Set the unique_event_id when converting from TouchEvent to WebTouchEvent

ui/events/gesture_detection/motion_event.h

Index: ui/events/gesture_detection/motion_event.h
diff --git a/ui/events/gesture_detection/motion_event.h b/ui/events/gesture_detection/motion_event.h
index 560afcbd044c20155ed281e7762f33ddbba77bb7..06e9d62a7a4a64f158f7d3ba633a8b5918e46824 100644
--- a/ui/events/gesture_detection/motion_event.h
+++ b/ui/events/gesture_detection/motion_event.h
@@ -46,7 +46,12 @@ class GESTURE_DETECTION_EXPORT MotionEvent {
 
   virtual ~MotionEvent() {}
 
+  // The touch ID of the first touch point of this motion event.
   virtual int GetId() const = 0;

[jdduke (slow), 2015/04/09 15:40:35]

Hmm, so why don't we just have a GetPointerId() accessor like we have GetX(),
GetY(), that just returns GetPointerId(0)?

+
+  // An unique identifier for the touch event which is converted to this motion
+  // event.

[tdresser, 2015/04/09 12:33:06]

This comment isn't quite right, because on Android, we start with a motion
event. 

I'd just say this is a unique identifier for this motion event.

[lanwei, 2015/04/16 17:09:06]

Done.

+  virtual uint64 GetUniqueEventId() const = 0;
   virtual Action GetAction() const = 0;
   // Only valid if |GetAction()| returns ACTION_POINTER_UP or
   // ACTION_POINTER_DOWN.