New static method I420Buffer::SetToBlack.

webrtc/media/base/fakevideorenderer.h

Index: webrtc/media/base/fakevideorenderer.h
diff --git a/webrtc/media/base/fakevideorenderer.h b/webrtc/media/base/fakevideorenderer.h
index 7398bbaa8bbf12d087b6bec7e5114c38ec9828c8..6b3780b3e3f26dfaa15d365fa5087edb904b04d8 100644
--- a/webrtc/media/base/fakevideorenderer.h
+++ b/webrtc/media/base/fakevideorenderer.h
@@ -34,7 +34,7 @@ class FakeVideoRenderer : public rtc::VideoSinkInterface<cricket::VideoFrame> {
     rtc::CritScope cs(&crit_);
     // TODO(zhurunz) Check with VP8 team to see if we can remove this

[perkj_webrtc, 2016/06/02 13:20:04]

remove this TODO if this works.

[nisse-webrtc, 2016/06/03 14:22:50]

I logged the largest Y value seen  supposedly black frames, and the largest
value seen when run rtc_media_unittests and peerconnection_unittests were 34.

So using 48 as the upper bound seems quite reasonable.

[pbos-webrtc, 2016/06/03 14:46:02]

That means that we cannot remove it right? Update the comment to say why we
permit non-exact Y?

[nisse-webrtc, 2016/06/08 09:25:28]

Done.

     // tolerance on Y values.
-    black_frame_ = CheckFrameColorYuv(6, 48, 128, 128, 128, 128, &frame);
+    black_frame_ = CheckFrameColorYuv(0, 48, 128, 128, 128, 128, &frame);
     // Treat unexpected frame size as error.
     ++num_rendered_frames_;
     width_ = frame.width();