Tidy up UserMediaController references from mediastream request objects.

Source/modules/mediastream/UserMediaRequest.cpp

 /*
  * Copyright (C) 2011 Ericsson AB. All rights reserved.
  * Copyright (C) 2012 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.
@@ skipping 161 matching lines @@
     ASSERT(!name.isEmpty());
     if (!executionContext())
         return;
     m_errorCallback->handleEvent(NavigatorUserMediaError::create(name, message, constraintName));
 }
 
 void UserMediaRequest::contextDestroyed()
 {
     if (m_controller) {
         m_controller->cancelUserMediaRequest(this);
-        m_controller = 0;
+        m_controller = nullptr;
     }
 
     ContextLifecycleObserver::contextDestroyed();
 }
 
 void UserMediaRequest::trace(Visitor* visitor)
 {
+    visitor->trace(m_controller);
     visitor->trace(m_successCallback);
     visitor->trace(m_errorCallback);
 }
 
 } // namespace blink