compositor: Wait for both Script and the Constellation when shutting down Pipelines (#37505)

Previously, the Constellation would immediately ask the Compositor to
shut down a pipeline, even before the ScriptThread finished shutting it
down. This meant that the Compositor might remove a Pipeline and then
re-add it if the ScriptThread sent a Pipeline-related message (such as a
new display list) in the meantime.

This change makes it so that the Compositor waits for both the
Constellation and the ScriptThread to finish shutting down a Pipeline
before removing its data. In addition, the Constellation no longer
synchronously waits on the Compositor when shutting down Pipelines. This
was important when the Compositor would talk to the ScriptThread
directly, but isn't necessary any longer.

Testing: This is very hard to test, because it depends on the creation
and destruction of many iframes and the particular timing of of all
the messaging between Servo bits. That said, this was tested manually
by observing the completion of Speedometer 2.1.
Fixes: #37458.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

components/shared/script/lib.rs

@@ -140,7 +140,7 @@ pub enum ScriptThreadMessage {
     /// Notifies the script that the document associated with this pipeline should 'unload'.
     UnloadDocument(PipelineId),
     /// Notifies the script that a pipeline should be closed.
-    ExitPipeline(PipelineId, DiscardBrowsingContext),
+    ExitPipeline(WebViewId, PipelineId, DiscardBrowsingContext),
     /// Notifies the script that the whole thread should be closed.
     ExitScriptThread,
     /// Sends a DOM event.