Tidy up UserMediaController references from mediastream request objects.

Source/modules/mediastream/MediaDevicesRequest.cpp

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 59 matching lines @@
 {
     if (!executionContext() || !m_callback)
         return;
 
     m_callback->handleEvent(mediaDevices);
 }
 
 void MediaDevicesRequest::stop()
 {
     m_callback.clear();
-    m_controller = 0;
+    m_controller.clear();
 }
 
 void MediaDevicesRequest::trace(Visitor* visitor)
 {
+    visitor->trace(m_controller);
     visitor->trace(m_callback);
 }
 
 } // namespace blink