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_) {
-    return true;
-  }
-  for (VideoRendererInterface* renderer : renderers_) {
-    renderer->RenderFrame(frame);
+  if (enabled_)
+    RenderFrameToRenderers(frame);
+  else {
+    cricket::WebRtcVideoFrame black(new rtc::RefCountedObject<I420Buffer>(
+                                        frame->GetWidth(), frame->GetHeight()),
+                                    frame->GetTimeStamp(),
+                                    frame->GetVideoRotation());

[pthatcher1, 2016/01/13 17:13:38]

Why does frame generation have to happen in the critical_section_?  Wouldn't it
make more sense to do it outside?

[nisse-webrtc, 2016/01/14 08:00:27]

Not sure, but I think it's desirable to have it consistent with the enabled
state. I.e., rule out the sequence

0. Start with enabled_ == false
1. Enter SetEnabled(true)
2. Enter RenderFrame. Still enabled_ == false, so start generating a black
frame.
3. The SetEnabled call sets enabled_ = true, and returns.
4. RenderFrame passes on the black frame to the renderers, even though the track
is supposedly enabled now.
 
But I see it's desirable to do the frame generation outside of the critical
section. It just needs a little more logic (after the frame is ready, enter the
critical section again, and recheck enabled_) to ensure consistency.

+    black.SetToBlack();
+    RenderFrameToRenderers(&black);
   }
   return true;
 }
 
+// Called with critical_section_ already locked
+void VideoTrackRenderers::RenderFrameToRenderers(
+    const cricket::VideoFrame* frame) {
+  for (VideoRendererInterface* renderer : renderers_) {
+    renderer->RenderFrame(frame);
+  }
+}
+
 }  // namespace webrtc