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/video_receiver.cc

@@ -335,6 +335,10 @@ int VideoReceiver::RegisterRenderBufferSizeCallback(
   return VCM_OK;
 }
 
+void VideoReceiver::TriggerDecoderShutdown() {
+  _receiver.TriggerDecoderShutdown();
+}
+
 // Decode next frame, blocking.
 // Should be called as often as possible to get the most out of the decoder.
 int32_t VideoReceiver::Decode(uint16_t maxWaitTimeMs) {