We currently might double-attach to the target in `BrowserHandler` since we iterate over all targets, and then subscribe to the additional event when target is getting initialized.
This patch fixes this race condition and should unblock the roll to r1177.
References #3995
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.