Cleanup of webrtc::VideoFrame.

Delete EqualsFrame method, used only by tests. Delete one of the
CreateFrame methods. Drop return value for CreateEmptyFrame, CreateFrame
and CopyFrame.

BUG=webrtc:5426

Review URL: https://codereview.webrtc.org/1679323002

Cr-Original-Commit-Position: refs/heads/master@{#11783}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 208019637bfed975f8f13b16d40b90e200763cd6