MediaCodecVideoEncoder: Add QP stats to Encoded callback for VP9 and turn on quality scaling.

Add default QP scaling thresholds for VP9.

BUG=webrtc:7662

Review-Url: https://codereview.webrtc.org/2914363002
Cr-Original-Commit-Position: refs/heads/master@{#18469}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 1e15a994ac668d37d8b2d7bbcf4864635e3fb3f3
4 files changed