Fixing a couple cases that cause ProcessAllMessageQueues to hang.

webrtc/base/messagehandler.h

 /*
  *  Copyright 2004 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 16 matching lines @@
   virtual ~MessageHandler();
   virtual void OnMessage(Message* msg) = 0;
 
  protected:
   MessageHandler() {}
 
  private:
   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 {
  public:
   explicit FunctorMessageHandler(const FunctorT& functor)
       : functor_(functor) {}
   virtual void OnMessage(Message* msg) {
     result_ = functor_();
   }
   const ReturnT& result() const { return result_; }
@@ skipping 28 matching lines @@
   }
   void result() const {}
 
  private:
   FunctorT functor_;
 };
 
 } // namespace rtc
 
 #endif // WEBRTC_BASE_MESSAGEHANDLER_H_