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The frame provided to the track is unchanged. Instead a black frame with the same timestamp is created and forwarded to the sinks.
Please see the bug and the discussion in https://groups.google.com/forum/#!topic/discuss-webrtc/nZWA0prohio for rational.

BUG=398844
TEST= open https://apprtc.appspot.com in two tabs to create a call. Hit Ctrl-shift-e in the first tab to disable the local video track. Notice that local video is black.  Open the second tab and notice that the remote video is black.

Review URL: https://codereview.chromium.org/432903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287787 0039d316-1c4b-4281-b951-d872f2087c98