Move BitrateAllocator from BitrateController logic to Call.

This is a step on the way to have variable bitrate for audio and is
intended to be as much of a no-op as possible.

BUG=webrtc:5079

Review URL: https://codereview.webrtc.org/1441673002

Cr-Original-Commit-Position: refs/heads/master@{#10630}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 0e7e259ebd69993bb5670a991f43aa1b06c9bf9e
18 files changed