Reason for revert:
Test has been fixed.
Original issue's description:
> Disable RTCVideoRenderFrameCallbackNV12 test
>
> It has been crashing on trybots
>
> BUG=webrtc:7863
> TBR=magjed@webrtc.org
> NOTRY=True
>
> Review-Url: https://codereview.webrtc.org/2951273002
> Cr-Commit-Position: refs/heads/master@{#18720}
> Committed: 4d25a0554a
TBR=kjellander@webrtc.org,oprypin@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:7863
Review-Url: https://codereview.webrtc.org/2959673002
Cr-Commit-Position: refs/heads/master@{#18758}