Set the unique_event_id when converting from TouchEvent to WebTouchEvent

ui/events/gesture_detection/motion_event.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_EVENTS_GESTURE_DETECTION_MOTION_EVENT_H_
 #define UI_EVENTS_GESTURE_DETECTION_MOTION_EVENT_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "base/time/time.h"
 #include "ui/events/gesture_detection/gesture_detection_export.h"
@@ skipping 28 matching lines @@
     BUTTON_BACK = 1 << 3,
     BUTTON_FORWARD = 1 << 4,
   };
 
   // The implementer promises that |GetPointerId()| will never exceed
   // MAX_POINTER_ID.
   enum { MAX_POINTER_ID = 31, MAX_TOUCH_POINT_COUNT = 16 };
 
   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.
   virtual int GetActionIndex() const = 0;
   virtual size_t GetPointerCount() const = 0;
   virtual int GetPointerId(size_t pointer_index) const = 0;
   virtual float GetX(size_t pointer_index) const = 0;
   virtual float GetY(size_t pointer_index) const = 0;
   virtual float GetRawX(size_t pointer_index) const = 0;
   virtual float GetRawY(size_t pointer_index) const = 0;
@@ skipping 44 matching lines @@
   // They guarantee only that the returned type will reflect the same
   // data exposed by the MotionEvent interface; no guarantees are made that the
   // underlying implementation is identical to the source implementation.
   scoped_ptr<MotionEvent> Clone() const;
   scoped_ptr<MotionEvent> Cancel() const;
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_GESTURE_DETECTION_MOTION_EVENT_H_