MediaStream: Futureproof browsertests
Adding support for the new MediaStream.clone API

BUG=294145

Review URL: https://codereview.chromium.org/94553002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237776 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237776