Drain compositor_chan on shutdown, causes deadlocks on constellation

Constellation blocks on SetId by sending a Port through the compositor
channel and waits for a response.  If the compositor is in the process of
shutting down, it will not look in the queue again.  The compositor
requires the constellation to be shut down first, so it sends a message
to shut down and blocks until the constellation finishes, deadlocking.

Only very short lived executions would've been likely to see this deadlock.

src/components/main/compositing/run.rs

@@ -421,4 +421,8 @@ pub fn run_compositor(compositor: &CompositorTask) {
         None => {}
         Some(ref mut layer) => layer.forget_all_tiles(),
     }
+
+    // Drain compositor port, sometimes messages contain channels that are blocking
+    // another task from finishing (i.e. SetIds)
+    while compositor.port.peek() { compositor.port.recv(); }
 }

src/components/main/constellation.rs

@@ -807,9 +807,13 @@ impl Constellation {
     fn set_ids(&self, frame_tree: @mut FrameTree) {
         let (port, chan) = comm::stream();
         self.compositor_chan.send(SetIds(frame_tree.to_sendable(), chan, self.chan.clone()));
-        port.try_recv();
-        for frame in frame_tree.iter() {
-            frame.pipeline.grant_paint_permission();
+        match port.try_recv() {
+            Some(()) => {
+                for frame in frame_tree.iter() {
+                    frame.pipeline.grant_paint_permission();
+                }
+            }
+            None => {} // message has been discarded, probably shutting down
         }
     }
 }