ViEFrameCallback::DeliverFrame: Make I420VideoFrame const ref.

This CL makes ViEFrameCallback::DeliverFrame const and removes the potential frame copy in ViEFrameProviderBase by moving it to ViEEncoder::DeliverFrame instead, for clients that use the FrameCallback functionality to modify the frame content.

BUG=1128
R=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/43949004

Cr-Commit-Position: refs/heads/master@{#8934}
10 files changed