fix: Stop throwing error when PointerEvent.pageX is 0. (google#5727)

The e.pageX (and .pageY) checks aren't strict enough.  0 is a perfectly valid coordinate.

Checking for the existence of changedTouches is an easier way to distinguish a PointerEvent from a TouchEvent.

core/touch_gesture.js

@@ -323,8 +323,8 @@ TouchGesture.prototype.getTouchPoint = function(e) {
     return null;
   }
   return new Coordinate(
-      (e.pageX ? e.pageX : e.changedTouches[0].pageX),
-      (e.pageY ? e.pageY : e.changedTouches[0].pageY));
+      (e.changedTouches ? e.changedTouches[0].pageX : e.pageX),
+      (e.changedTouches ? e.changedTouches[0].pageY : e.pageY));
 };
 
 exports.TouchGesture = TouchGesture;