Coalesce async touch move events until the ack back from render

ui/events/event.h

Index: ui/events/event.h
diff --git a/ui/events/event.h b/ui/events/event.h
index e1ca9747b0b1940eecf091ae4b01a841528ec432..30d6cb56f6877548f9156038b69b4499455c12a6 100644
--- a/ui/events/event.h
+++ b/ui/events/event.h
@@ -518,7 +518,7 @@ class EVENTS_EXPORT TouchEvent : public LocatedEvent {
   // The id of the pointer this event modifies.
   int touch_id() const { return touch_id_; }
   // A unique identifier for this event.
-  uint64 unique_event_id() const { return unique_event_id_; }
+  uint32 unique_event_id() const { return unique_event_id_; }
   // If we aren't provided with a radius on one axis, use the
   // information from the other axis.
   float radius_x() const { return radius_x_ > 0 ? radius_x_ : radius_y_; }
@@ -558,7 +558,7 @@ class EVENTS_EXPORT TouchEvent : public LocatedEvent {
   const int touch_id_;
 
   // A unique identifier for the touch event.
-  const uint64 unique_event_id_;
+  const uint32 unique_event_id_;
 
   // Radius of the X (major) axis of the touch ellipse. 0.0 if unknown.
   float radius_x_;