WebRTC no longer has any restriction on what thread frames should be
delivered on. One possible problem with this CL is that NV21->I420
conversion and scaling is done on the thread that delivers frames, which
might cause fps regressions.
R=nisse@webrtc.org, perkj@webrtc.org, tkchin@webrtc.org
Review URL: https://codereview.webrtc.org/2137503003 .
Cr-Commit-Position: refs/heads/master@{#14021}
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