Signal threads for faster receiver destruction.

Unblocks pending threads (render thread + decoder thread) when
destroying renderers and shutting down decoders.

Speeds up SetLocalDescription significantly (10x or so) under
WebRtcVideoEngine2 but also shutdown times in ~ViEChannel and
~ViEReceiver in general.

BUG=1788
R=mflodman@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/41959004

Cr-Commit-Position: refs/heads/master@{#8387}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8387 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/video_coding/main/source/receiver.h

@@ -81,11 +81,13 @@ class VCMReceiver {
 
   void RegisterStatsCallback(VCMReceiveStatisticsCallback* callback);
 
+  void TriggerDecoderShutdown();
+
  private:
   static int32_t GenerateReceiverId();
 
   CriticalSectionWrapper* crit_sect_;
-  Clock* clock_;
+  Clock* const clock_;
   VCMJitterBuffer jitter_buffer_;
   VCMTiming* timing_;
   scoped_ptr<EventWrapper> render_wait_event_;