This CL is broken out from a future "real" CL, that introduces
support for pipelining HW codecs to VideoProcessor. I order to
simplify the reviewing of that CL a bit, some of the cleanups are
split out here.
No functional changes are intended.
BUG=webrtc::6634
Review-Url: https://codereview.webrtc.org/2709123004
Cr-Commit-Position: refs/heads/master@{#16909}