Fixing a couple cases that cause ProcessAllMessageQueues to hang.

webrtc/base/messagehandler.h

Index: webrtc/base/messagehandler.h
diff --git a/webrtc/base/messagehandler.h b/webrtc/base/messagehandler.h
index 6a3c2ef7402bdf9203ef350dcff740f806d1210d..914d1797f1d5b297e52f8297884506ef3d233baf 100644
--- a/webrtc/base/messagehandler.h
+++ b/webrtc/base/messagehandler.h
@@ -34,6 +34,11 @@ class MessageHandler {
   RTC_DISALLOW_COPY_AND_ASSIGN(MessageHandler);
 };
 
+class NoOpMessageHandler : public MessageHandler {
+ public:
+  void OnMessage(Message* msg) override;
+};
+
 // Helper class to facilitate executing a functor on a thread.
 template <class ReturnT, class FunctorT>
 class FunctorMessageHandler : public MessageHandler {