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/interface/video_coding.h

@@ -582,6 +582,8 @@ public:
         EncodedImageCallback* observer) = 0;
     virtual void RegisterPostEncodeImageCallback(
         EncodedImageCallback* post_encode_callback) = 0;
+    // Releases pending decode calls, permitting faster thread shutdown.
+    virtual void TriggerDecoderShutdown() = 0;
 };
 
 }  // namespace webrtc