Add disable-avfoundation flag to chrome://flags

BUG=345081, 288562

Review URL: https://codereview.chromium.org/171533016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252446 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252446