RentACodec: New class that takes over part of ACMCodecDB's job

Following CLs will finish the takeover completely. After that,
RentACodec will also start creating and owning codecs, at which point
its name will start making sense.

BUG=webrtc:5028

Review URL: https://codereview.webrtc.org/1412683006

Cr-Original-Commit-Position: refs/heads/master@{#10432}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: fce4a945b872e4ca52e76a3872a3278a88e17b85
11 files changed