browser(firefox): use browsingContextID for frame IDs

[aslushnikov]

BrowsingContextIDs are consistent across the processes, so we can use
them to target frames in both browser and content processes. This will
aid browser-side navigation.

As a nice side-effect, we can drop a round-trip to the content process
for every requestWillBeSent event since we almost always can
attribute all network events to the proper parent frames.

I say "almost", because we in fact fail to correctly attribute requests
from workers that are instantiated by subframes. This, however, is
not working in Chromium ATM, so I consider this to be a minor regression
that is worth the simplification.

[dgozman]

This looks great! Do we have tests for network in popups? I am a bit worried that browsingContext might not be there in time, otherwise everything looks perfect.

[aslushnikov]

This looks great! Do we have tests for network in popups? I am a bit worried that browsingContext might not be there in time, otherwise everything looks perfect.

There's a test for request interception for the popup document request - and it works. There's no route.request().frame() though since there's no frame tree to report until the popup is starting loading - but this is consistent with current behavior.