Fix/Add unittest to work for the code behind --enable-audio-track-processing.



BUG=264611
TEST=content_unittests

Review URL: https://codereview.chromium.org/190643003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256640 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256640