This patch:
1. Changes `SimpleChannel` to buffer messages to the namespace that
hasn't been registered yet. This allows us to create `SimpleChannel`
per target on the browser side right away.
2. Removes multisession support. Now there's only one `PageAgent` in the
content process, which talks to a single `PageHandler` on the browser
side. Both ends can be created as-soon-as-needed; thanks to
`SimpleChannel` bufferring, no messages will be lost and all messages
will be delivered in proper order. (This is currently the reason why
build 1178 flakes on windows).
3. Straightens up the target reporting. Targets are reported as soon
as they appear on the browser side.
**NOTE:** this doesn't yet remove sessions from protocol.
References #3995
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.
