Because there used to be two traits exposing messages to the compositor,
there were two kinds of messages that could be sent:
1. In-process messages from the `Constellation`
2. Cross-process messages from other parts of Servo
Now these two types of messages can be unified into one type. With that
done the compositor can simply keep a single `IpcReceiver` for all
messages, instead of having to set up a route for the cross-process
messsages. This decreases overhead of cross proceses messages a bit, but
more importantly solves an issue where Servo would rely on the
compositor's cross-process message route after the `Constellation` had
called `ROUTER.shutdown()`.
This is part of #36442.
Testing: This is covered by existing WPT tests.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
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