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
diff --git a/talk/app/webrtc/videosourceinterface.h b/talk/app/webrtc/videosourceinterface.h
index a90e3d5..2b3a2e6 100644
--- a/talk/app/webrtc/videosourceinterface.h
+++ b/talk/app/webrtc/videosourceinterface.h
@@ -45,8 +45,9 @@
   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;