Added new entry to the Blink.MediaElement.Autoplay histogram to account for cases where autoplay is reenabled on a Media element by a user-gesture triggered script

R=qinmin,adamk
BUG=432734

Review URL: https://codereview.chromium.org/719413002

git-svn-id: svn://svn.chromium.org/blink/trunk@185332 bbb929c8-8fbe-4397-9dbb-9b2b20218538