Added UMAs needed for media autoplay metrics

We count:
* Total number of autoplay videos encountered (started via the autoplay attribute, or the play() function)
* Total number of times a user bails out on a video autoplayed due to gestures not required
* Total number of times a user manually starts a video which would've autoplayed if gestures weren't required

R=alexei,dalecurtis,adamk
TBR=philipj
BUG=402044

Review URL: https://codereview.chromium.org/697723004

git-svn-id: svn://svn.chromium.org/blink/trunk@184928 bbb929c8-8fbe-4397-9dbb-9b2b20218538