Remove lastHandledUserGestureTimestamp

Nothing reads this value, we don't need to keep updating it and calling
currentTime(). This also lets us remove the userEventWasHandled() client
callback since no one needs it now.

Review URL: https://codereview.chromium.org/225093003

git-svn-id: svn://svn.chromium.org/blink/trunk@170826 bbb929c8-8fbe-4397-9dbb-9b2b20218538