Add default implementation to VideoSourceInterface of Stop and Restart.

This is to make sure Chrome does not break when rolling. This should be reverted once
Chrome has been updated.

Please see:
http://build.chromium.org/p/chromium.webrtc.fyi/builders/Mac/builds/16556/steps/compile/logs/stdio

BUG=4303
R=magjed@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/35229004

Cr-Commit-Position: refs/heads/master@{#8391}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8391 4adac7df-926f-26a2-2b94-8c16560cd09d

talk/app/webrtc/videosourceinterface.h

@@ -45,8 +45,9 @@ class VideoSourceInterface : public MediaSourceInterface {
   virtual cricket::VideoCapturer* GetVideoCapturer() = 0;
 
   // Stop the video capturer.
-  virtual void Stop() = 0;
-  virtual void Restart() = 0;
+  // TODO(perkj): Make pure virtual after updating Chrome.
+  virtual void Stop() {};
+  virtual void Restart() {};
 
   // Adds |output| to the source to receive frames.
   virtual void AddSink(cricket::VideoRenderer* output) = 0;