Cleanups in cricket::VideoFrame and cricket::WebRtcVideoFrame.

Removed some protected virtual methods from VideoFrame that no longer
need to exist. Some minor cleanups in the tests.

BUG=webrtc:5682

Review-Url: https://codereview.webrtc.org/2075983003
Cr-Original-Commit-Position: refs/heads/master@{#13275}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 742d7b10b9720ec43de26e0faef52e5cb9c0daa8
5 files changed