We missed to implement quality scaling in the original CL
https://codereview.webrtc.org/2977213002/. This CL implements it.
Note that the ObjC interface for scalingSettings is slightly different from the C++
interface in that we require explicit QP thresholds to turn quality scaling on, i.e.
we don't provide default values. I think this is more modular as we want to move
codec specific knowledge out from the WebRTC core. I would like to update the
C++ webrtc::VideoEncoder interface to do the same in another CL.
BUG=webrtc:7924
Review-Url: https://codereview.webrtc.org/2991123002
Cr-Commit-Position: refs/heads/master@{#19202}
5dfac33dfd