Move BitrateAdjuster into common_video

This CL moves BitrateAdjuster into common_video folder as it
was suggested on [0] such that it can be properly linked with
Chrome projects.

[0] https://codereview.chromium.org/1818903004/

BUG=500605

Review URL: https://codereview.webrtc.org/1914893005

Cr-Original-Commit-Position: refs/heads/master@{#12515}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 55a401e607dfdca2f40c7b912c11a9c7e6b10f86
10 files changed