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-Commit-Position: refs/heads/master@{#12515}

webrtc/modules/video_coding/video_coding.gypi

@@ -22,7 +22,6 @@
       ],
       'sources': [
         # interfaces
-        'include/bitrate_adjuster.h',
         'include/video_coding.h',
         'include/video_coding_defines.h',
 
@@ -60,7 +59,6 @@
         'video_coding_impl.h',
 
         # sources
-        'bitrate_adjuster.cc',
         'codec_database.cc',
         'codec_timer.cc',
         'content_metrics_processing.cc',