Comment stating when to set tap details of WebGestureEvent.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@169133 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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