Use bitstream-level QP for libvpx VP8 quality.
Use bitstream-level QP for libvpx VP8 quality.

BUG=webrtc:5678
TBR=marpan@webrtc.org

Review URL: https://codereview.webrtc.org/1888843002 .

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#12401}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8056acc6f5c4b4d005ca3c77fb6d6f109632cebc

Alkalyne/webrtctrunk,jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk