ReAdd dummy MediaStreamTrack::set_state to make Chrome build happy.

https://codereview.webrtc.org/1816143002/ broke the Chrome test builds. The method is only used for tests and adding a dummy implementation seem to be enough.

BUG=webrtc:5426
R=guidou@webrtc.org

Review URL: https://codereview.webrtc.org/1828233002 .

Cr-Commit-Position: refs/heads/master@{#12118}

webrtc/api/mediastreaminterface.h

@@ -92,6 +92,8 @@ class MediaStreamTrackInterface : public rtc::RefCountInterface,
   virtual bool enabled() const = 0;
   virtual TrackState state() const = 0;
   virtual bool set_enabled(bool enable) = 0;
+  // TODO(perkj): Remove when Chrome tests doesn't rely on this method.
+  virtual bool set_state(TrackState state) { return true;}
 
  protected:
   virtual ~MediaStreamTrackInterface() {}