Comment stating when to set tap details of WebGestureEvent.

Added a comment describing that the tap field of WebGestureEvent
must be set for GestureTap, GestureTapUnconfirmed, and
GestureDoubleTap events.

BUG=351793

Review URL: https://codereview.chromium.org/199023002

git-svn-id: svn://svn.chromium.org/blink/trunk@169133 bbb929c8-8fbe-4397-9dbb-9b2b20218538