libvpx flags aren't applicable to hardware encoders or non-libvpx
software encoders. This moves libvpx flag conversion into the
VP8EncoderImpl integration.
BUG=chromium:702017,webrtc:7349
R=brandt@webrtc.org
Review-Url: https://codereview.webrtc.org/2849723002
Cr-Commit-Position: refs/heads/master@{#17970}