Cherry pick: VP8: For high overshoot, force drop frame and max-out QP.

VP8: For high overshoot, force drop frame and max-out QP.

This allows rate control to react to content of current frame being encoded.
Enabling this feature via the setting: screen_content_mode = 2.

https://chromium-review.googlesource.com/#/c/267789/

R=tomfinegan@chromium.org
TBR=tomfinegan@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1160103002
6 files changed