gesture: Set the |tapCount| when converting from WebCore::GestureEvent to WebGestureEvent.

When gesture events are converted from PlatformGestureEvent to WebCore::GestureEvent, we lose some
information about the events (e.g. tap-count for the gesture). So make sure we set the tap-count to
1 for tap-gestures. This is a partial fix. The proper fix is tracked in crbug.com/332001

BUG=330995
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/125643003

git-svn-id: svn://svn.chromium.org/blink/trunk@164589 bbb929c8-8fbe-4397-9dbb-9b2b20218538