In the current tip-of-tree Firefox, document channel is enabled by
default, so we have to enable it in order to roll further.
This patch:
1. Removes content disposition sniffing from content process since it
crashes renderer with document channel.
2. Merges all page-related handlers in a single `PageHandler` and
serializes network events wrt the `Page.frameAttached` event.
The serialization mentioned in (2) is necessary: frame attachment is
reported from the content process, and network events are reported from
the browsers process. This is an inherent race, that becomes exposed by
the document channel.
On a side note, (2) makes it possible to synchronously report all
buffered events in `SimpleChannel` (cc offline discussion with @dgozman
that highlighted an unsighty approach that we currently employ there: reporting
events in a subsequent microtask.)
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.
