Moves towards separating which layers may be referenced instead of
referencing libvpx flags directly. This will make strategies easier to
extract and usable from hardware encoders (RTCVideoEncoder, for
instance).
BUG=chromium:702017, webrtc:7349
R=brandtr@webrtc.org, marpan@webrtc.org. sprang@webrtc.org
Review-Url: https://codereview.webrtc.org/2747123005
Cr-Commit-Position: refs/heads/master@{#17349}
e6fe2a9a7f