Don't use VideoFrameBuffer::MutableDataY and friends, instead, use I420Buffer::SetToBlack. Also introduce static method I420Buffer::Create, to create an object and return a scoped_refptr. TBR=marpan@webrtc.org # Trivial change to video_denoiser.cc BUG=webrtc:5921 Review-Url: https://codereview.webrtc.org/2078943002 Cr-Commit-Position: refs/heads/master@{#13212}