Revert "GenericChannel: Migrate compositor channels to GenericChannel (#38782)" (#38940)

This reverts commit fb1c0a4c48.

Previously in `create_compositor_channel`, the [routing callback][1] was
setup so that a message received on the Compositor's IPC receiver will
be
forwarded to the local receiver using the `CompositorProxy` which also
takes care of waking up the event loop. In #38782, this was changed so
that the routing callbacks simply forwards the message directly without
going via the `CompositorProxy`. This breaks behaviours that rely on the
event loop being woken up on message sending, e.g. updating image frames
for animated gifs.

Since the GenericChannel API doesn't allow custom routing callbacks,
revert this change until we figure out a better solution.

[1]:
d2ccce6052/components/servo/lib.rs (L1114)

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

components/script/messaging.rs

@@ -340,7 +340,7 @@ pub(crate) struct ScriptThreadSenders {
     /// particular pipelines.
     #[no_trace]
     pub(crate) pipeline_to_constellation_sender:
-        GenericSender<(PipelineId, ScriptToConstellationMessage)>,
+        IpcSender<(PipelineId, ScriptToConstellationMessage)>,
 
     /// The shared [`IpcSender`] which is sent to the `ImageCache` when requesting an image. The
     /// messages on this channel are routed to crossbeam [`Sender`] on the router thread, which