Various fixes for cleaning up iframes, compositor layers, pipelines and threads.

This allows most of the jquery test suite to run without exhausting thread resources.

components/msg/constellation_msg.rs

@@ -244,6 +244,8 @@ pub enum Msg {
     CompositePng(Sender<Option<png::Image>>),
     /// Query the constellation to see if the current compositor output is stable
     IsReadyToSaveImage(HashMap<PipelineId, Epoch>),
+    /// Notification that this iframe should be removed.
+    RemoveIFrame(PipelineId, SubpageId),
 }
 
 #[derive(Clone, Eq, PartialEq)]