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.
This is a reland of #36443, which caused regressions due to the fact
that messages to the compositor were no longer triggering the event loop
waker. This version of the PR splits out just the bits that unify the
two APIs, leaving the cleanup of routes in the constellation for another
PR.
Testing: This is covered by existing WPT tests.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
e9daec7d42