MediaStream API: Adding the last of the Clone functionality
This Patch has been in twice and rolled out due to bugs found in the
underlying implementation but I have been assured that they have now been
fixed. Since so much has happened in the code base it wasn't posible to
simply revert the revert.

BUG=294145

Review URL: https://codereview.chromium.org/184293006

git-svn-id: svn://svn.chromium.org/blink/trunk@169015 bbb929c8-8fbe-4397-9dbb-9b2b20218538