Fix some double-counting of Blink.MediaElement.Autoplay histogram entries
Fix some double-counting of Blink.MediaElement.Autoplay histogram entries

Also included some more "stop" scenarios (like setting the 'src' attribute).

R=qinmin,adamk
BUG=432734

Review URL: https://codereview.chromium.org/754803003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@185974 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs