Store the id of a contributing motion event in GestureEventData

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 11 matching lines @@
     ACTION_CANCEL,
     ACTION_POINTER_DOWN,
     ACTION_POINTER_UP,
   };
 
   // The implementer promises that |GetPointerId()| will never exceed this.
   enum { MAX_POINTER_ID = 31 };
 
   virtual ~MotionEvent() {}
 
+  virtual int GetId() const = 0;
   virtual Action GetAction() const = 0;
   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 GetTouchMajor(size_t pointer_index) const = 0;
   virtual float GetPressure(size_t pointer_index) const = 0;
   virtual base::TimeTicks GetEventTime() const = 0;
 
@@ skipping 15 matching lines @@
   float GetY() const { return GetY(0); }
   float GetRawX() const { return GetX(); }
   float GetRawY() const { return GetY(); }
   float GetTouchMajor() const { return GetTouchMajor(0); }
   float GetPressure() const { return GetPressure(0); }
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_GESTURE_DETECTION_MOTION_EVENT_H_