Remove WebInputEvent::isUserGestureEventType

git grep tells me this method is not used anywhere.

BUG=337429

Review URL: https://codereview.chromium.org/258733004

git-svn-id: svn://svn.chromium.org/blink/trunk@172610 bbb929c8-8fbe-4397-9dbb-9b2b20218538