Support encoding from CVPixelBuffers directly in H264VideoToolboxEncoder.
If the frame needs to be scaled, it will fall back to the previous slow
path:
CVPixelBuffer -> NV12 -> I420 -> (scale) I420 -> NV12 -> CVPixelBuffer.
BUG=webrtc:4081
Review-Url: https://codereview.webrtc.org/2140573002
Cr-Commit-Position: refs/heads/master@{#13439}