Added mute logic to VideoTrackRenderers.

talk/app/webrtc/videotrackrenderers.cc

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 #include "talk/app/webrtc/videotrackrenderers.h"
-#include "talk/media/base/videoframe.h"
+#include "talk/media/webrtc/webrtcvideoframe.h"
 
 namespace webrtc {
 
 VideoTrackRenderers::VideoTrackRenderers() : enabled_(true) {
 }
 
 VideoTrackRenderers::~VideoTrackRenderers() {
 }
 
 void VideoTrackRenderers::AddRenderer(VideoRendererInterface* renderer) {
   if (!renderer) {
     return;
   }
   rtc::CritScope cs(&critical_section_);
   renderers_.insert(renderer);
 }
 
 void VideoTrackRenderers::RemoveRenderer(VideoRendererInterface* renderer) {
   rtc::CritScope cs(&critical_section_);
   renderers_.erase(renderer);
 }
 
 void VideoTrackRenderers::SetEnabled(bool enable) {
   rtc::CritScope cs(&critical_section_);
   enabled_ = enable;
 }
 
 bool VideoTrackRenderers::RenderFrame(const cricket::VideoFrame* frame) {
-  rtc::CritScope cs(&critical_section_);
-  if (!enabled_) {
+  {
+    rtc::CritScope cs(&critical_section_);

[pthatcher, 2016/01/14 19:23:09]

Can you put a comment that this may result in out-of-order frames if RenderFrame
is called on different threads and SetEnabled(true) is called right in the
middle of two RenderFrame calls?

It likely doesn't matter at all since it's highly unlikely we'll be producing
frames on different threads, but it's probably worth leaving a warning/comment.

+    if (enabled_) {
+      RenderFrameToRenderers(frame);
+      return true;
+    }
+  }
+
+  // Generate the black frame outside of the critical section.
+  cricket::WebRtcVideoFrame black(new rtc::RefCountedObject<I420Buffer>(
+                                      frame->GetWidth(), frame->GetHeight()),
+                                  frame->GetTimeStamp(),
+                                  frame->GetVideoRotation());
+  black.SetToBlack();
+
+  {
+    rtc::CritScope cs(&critical_section_);
+    // Check enabled_ flag again, since the track might have been
+    // enabled while we generated the black frame. I think the
+    // enabled-ness ought to be applied at the track output, and hence
+    // an enabled track shouldn't send any blacked out frames.
+    RenderFrameToRenderers(enabled_ ? frame : &black);
+
     return true;
   }
+}
+
+// Called with critical_section_ already locked
+void VideoTrackRenderers::RenderFrameToRenderers(
+    const cricket::VideoFrame* frame) {
   for (VideoRendererInterface* renderer : renderers_) {
     renderer->RenderFrame(frame);
   }
-  return true;
 }
 
 }  // namespace webrtc