Move the QP scaling thresholds to the relevant encoders.

Also provide a new set of thresholds for the VideoToolbox encoder. The new thresholds were experimentally determined to work well on the iPhone 6S, and also adequately on the iPhone 5S.

BUG=webrtc:5678

Review-Url: https://codereview.webrtc.org/2309743002
Cr-Original-Commit-Position: refs/heads/master@{#14420}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 478681e1e6e6c5d66366672614d58b10ad7df083
6 files changed