Replacing the 'return 0;' statements with 'return nullptr;'

public/platform/WebMediaStreamCenter.h

 /*
  * 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 38 matching lines @@
     virtual bool didRemoveMediaStreamTrack(const WebMediaStream&, const WebMediaStreamTrack&) = 0;
     virtual void didStopLocalMediaStream(const WebMediaStream&) = 0;
 
     // Track functionality.
     virtual void didCreateMediaStreamTrack(const WebMediaStreamTrack&) { }
     virtual void didEnableMediaStreamTrack(const WebMediaStreamTrack&) { }
     virtual void didDisableMediaStreamTrack(const WebMediaStreamTrack&) { }
     virtual bool didStopMediaStreamTrack(const WebMediaStreamTrack&) { return false; }
 
     // Caller must take the ownership of the returned |WebAudioSourceProvider| object.
-    virtual WebAudioSourceProvider* createWebAudioSourceFromMediaStreamTrack(const WebMediaStreamTrack&) { return 0; }
+    virtual WebAudioSourceProvider* createWebAudioSourceFromMediaStreamTrack(const WebMediaStreamTrack&) { return nullptr; }
 };
 
 } // namespace blink
 
 #endif // WebMediaStreamCenter_h