compositor: Unify the cross process and in-process API (#36443)

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>

components/script/dom/window.rs

@@ -1898,10 +1898,7 @@ impl Window {
         let (sender, receiver) =
             ProfiledIpc::channel::<DeviceIndependentIntRect>(timer_profile_chan).unwrap();
         let _ = self.compositor_api.sender().send(
-            compositing_traits::CrossProcessCompositorMessage::GetClientWindowRect(
-                self.webview_id(),
-                sender,
-            ),
+            compositing_traits::CompositorMsg::GetClientWindowRect(self.webview_id(), sender),
         );
         let rect = receiver.recv().unwrap_or_default();
         (