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.
