Add showPress data to WebGestureEvent.

BUG=307485
TEST=WebInputEventConversionTest.InputEventsScaling

Review URL: https://codereview.chromium.org/27043005

git-svn-id: svn://svn.chromium.org/blink/trunk@159744 bbb929c8-8fbe-4397-9dbb-9b2b20218538