Require system focus for gamepad user gesture

[nondebug]

Closes #206

The following tasks have been completed:

• Modified Web platform tests (link to pull request)

Implementation commitment:

• WebKit
• Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
• Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)

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Preview | Diff

[marcoscaceres]

This seems reasonable. I need to check if WebKit already does this or not.

[nondebug]

This seems reasonable. I need to check if WebKit already does this or not.

I think WebKit on macOS already requires focus even for normal gamepad inputs because it uses Game Controller API which requires focus.

[marcoscaceres]

We might actually want this:
https://html.spec.whatwg.org/multipage/interaction.html#fully-active-descendant-of-a-top-level-traversable-with-user-attention

• Fully active
• visibly requirement
• and system system focus!