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This allows VP9/lossy mode to "top-off" previously encoded imagery to
improve visual quality at the client, by continuing to deliver data even
when nothing has actually changed on the host desktop.

This CL makes several other minor changes:
- Fix the host's --enable-i444 flag, which was broken by refactoring.
- Tweak VP9/lossy mode to more conservative quantization settings.
- Tweak VP9/lossy mode to encode up to two unchanged frames for top-off.
- Update VideoEncoderVerbatim for the new VideoFramePump semantics.
- Update the VideoFrameRecorder & FakeDesktopCapturer.
- Adds simple codec & VideoFramePump unit-tests for the new behaviour.

BUG=134202

Review URL: https://codereview.chromium.org/1150163002

Cr-Commit-Position: refs/heads/master@{#333764}