Populates sourceDevice attribute into TouchEvent

Source/core/input/InputDevice.idl

 // Copyright 2015 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.
 
 // Represents an input device (or group of related devices).
 // To ease implementation, developers cannot rely on comparing two InputDevice
 // instances for equality. Now we only have two InputDevice instances: one
 // represents all the devices which send touch events, the other one covers all
 // other devices which do not send touch events, because currently all we care
 // about is if the input device fires touch events or not.
 [
     Constructor(optional InputDeviceInit deviceInitDict),
     RuntimeEnabled=InputDevice,
-    GarbageCollected
+    GarbageCollected,
 ] interface InputDevice {
     
     // Whether this device dispatches touch events for movement.  This is used to detect
     // mouse events which represent only an action that has already been handled  by
     // touch event handlers.
     readonly attribute boolean firesTouchEvents;
-};
+};