Bug 1389021 - Explicitly shutdown the CompositorManagerChild during ContentChild::ActorDestroy. r=me

It has been observed on nightly and beta that the compositer thread
fails to shutdown gracefully due to lingering references. From what
can be determined, it appears as if the content process references
are what are keeping it alive. The shutdown of CompositorBridgeChild
was altered because a top level protocol was added above it in a
previous change in bug 1365927. This protocol tree is ultimately
what is keeping the thread alive. As such, this patch adds an
explicit shutdown of the protocol, to ensure it gets released in a
timely manner.

This change will be backed out if it appears to have no effect on
the crash rate in nightly 57.

dom/ipc/ContentChild.cpp

@@ -2310,6 +2310,8 @@ ContentChild::ActorDestroy(ActorDestroyReason why)
   }
 
 #ifndef NS_FREE_PERMANENT_DATA
+  CompositorManagerChild::Shutdown();
+
   // In release builds, there's no point in the content process
   // going through the full XPCOM shutdown path, because it doesn't
   // keep persistent state.