Clarify target object of Client.postMessage can be null (#1274)

This change addresses the issue described in the second paragraph of
#1042 (comment).
This change gets |destination| variable initialized to null if the
ServiceWorkerContainer object does not exist.

Fixes #1042.

Author: jungkees

docs/index.bs

@@ -1028,7 +1028,7 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
       The <dfn method for="Client"><code>postMessage(|message|, |transfer|)</code></dfn> method *must* run these steps:
 
         1. Let |sourceSettings| be the <a>context object</a>'s <a>relevant settings object</a>.
-        1. Let |destination| be the {{ServiceWorkerContainer}} object whose [=ServiceWorkerContainer/service worker client=] is the <a>context object</a>'s [=Client/service worker client=].
+        1. Let |destination| be the {{ServiceWorkerContainer}} object whose [=ServiceWorkerContainer/service worker client=] is the <a>context object</a>'s [=Client/service worker client=], or null if no match is found.
         1. If |destination| is null, <a>throw</a> an "{{InvalidStateError}}" {{DOMException}}.
         1. Let |serializeWithTransferResult| be <a abstract-op>StructuredSerializeWithTransfer</a>(|message|, |transfer|). Rethrow any exceptions.
         1. Add a <a>task</a> that runs the following steps to |destination|'s [=ServiceWorkerContainer/client message queue=]:
@@ -1209,7 +1209,7 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
                     1. Else, set |browsingContext| to |client|’s [=environment/target browsing context=].
                     1. [=Queue a task=] |task| to run the following substeps on |browsingContext|'s [=event loop=] using the [=user interaction task source=]:
                         1. If |browsingContext| has been [=a browsing context is discarded|discarded=], then set |isClientEnumerable| to false and abort these steps.
-                        1. If |client| is a window client and |client|'s [=responsible document=] is not |browsingContext|'s [=active document=],  then set |isClientEnumerable| to false and abort these steps.
+                        1. If |client| is a window client and |client|'s [=responsible document=] is not |browsingContext|'s [=active document=], then set |isClientEnumerable| to false and abort these steps.
                         1. Set |windowData|["`visibilityState`"] to |browsingContext|'s [=active document=]'s {{Document/visibilityState}} attribute value.
                         1. Set |windowData|["`focusState`"] to the result of running the [=has focus steps=] with |browsingContext|'s [=active document=] as the argument.
                         1. If |client| is a [=window client=], then set |windowData|["`ancestorOriginsList`"] to |browsingContext|'s [=active document=]'s [=relevant global object=]'s {{Location}} object's [=Location/ancestor origins list=]'s associated list.

docs/v1/index.bs

@@ -966,7 +966,7 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
       The <dfn method for="Client"><code>postMessage(|message|, |transfer|)</code></dfn> method *must* run these steps:
 
         1. Let |sourceSettings| be the <a>context object</a>'s <a>relevant settings object</a>.
-        1. Let |destination| be the {{ServiceWorkerContainer}} object whose [=ServiceWorkerContainer/service worker client=] is the <a>context object</a>'s [=Client/service worker client=].
+        1. Let |destination| be the {{ServiceWorkerContainer}} object whose [=ServiceWorkerContainer/service worker client=] is the <a>context object</a>'s [=Client/service worker client=], or null if no match is found.
         1. If |destination| is null, <a>throw</a> an "{{InvalidStateError}}" {{DOMException}}.
         1. Let |serializeWithTransferResult| be <a abstract-op>StructuredSerializeWithTransfer</a>(|message|, |transfer|). Rethrow any exceptions.
         1. Add a <a>task</a> that runs the following steps to |destination|'s [=ServiceWorkerContainer/client message queue=]:
@@ -1147,7 +1147,7 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
                     1. Else, set |browsingContext| to |client|’s [=environment/target browsing context=].
                     1. [=Queue a task=] |task| to run the following substeps on |browsingContext|'s [=event loop=] using the [=user interaction task source=]:
                         1. If |browsingContext| has been [=a browsing context is discarded|discarded=], then set |isClientEnumerable| to false and abort these steps.
-                        1. If |client| is a window client and |client|'s [=responsible document=] is not |browsingContext|'s [=active document=],  then set |isClientEnumerable| to false and abort these steps.
+                        1. If |client| is a window client and |client|'s [=responsible document=] is not |browsingContext|'s [=active document=], then set |isClientEnumerable| to false and abort these steps.
                         1. Set |windowData|["`visibilityState`"] to |browsingContext|'s [=active document=]'s {{Document/visibilityState}} attribute value.
                         1. Set |windowData|["`focusState`"] to the result of running the [=has focus steps=] with |browsingContext|'s [=active document=] as the argument.
                         1. If |client| is a [=window client=], then set |windowData|["`ancestorOriginsList`"] to |browsingContext|'s [=active document=]'s [=relevant global object=]'s {{Location}} object's [=Location/ancestor origins list=]'s associated list.