Remove dependency on Chrome ScriptID

[grouma]

As of #1078 we no longer require the scriptID to set breakpoints. However, we still depend on the scriptID provided in the JS frames to translate them to Dart frames. However, the script URL is also provided in the JS frames which should provide enough information to do the translation. Removing our dependency on the script ID will greatly improve the IPL of Dart DevTools as we will no longer saturate the Debug Extension connection with these ScriptParsed events.