b/3350774 avoid screen jumping when pinching to zoom.

After the change that sends multi-touch to WebKit first, there is
a chance that the single-touch move event is handled before multi-
touch events are finished and this makes the page jumps.
This change skips those single-touch move events to avoid that.

Change-Id: I03992ef18c1de8e07933bd7cc6c12ffc02c014a8