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
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
