Create movementX/Y on MouseEvent

Source/core/events/MouseEvent.idl

Index: Source/core/events/MouseEvent.idl
diff --git a/Source/core/events/MouseEvent.idl b/Source/core/events/MouseEvent.idl
index 16beaeb99dfb6cbb9655ac99e6fe5dd4c34e14dd..f5b03c862d9ef20bacdf38f6dd2d5f178741c9c3 100644
--- a/Source/core/events/MouseEvent.idl
+++ b/Source/core/events/MouseEvent.idl
@@ -30,8 +30,10 @@
     [InitializedByEventConstructor] readonly attribute boolean          metaKey;
     [InitializedByEventConstructor] readonly attribute unsigned short   button;
     [InitializedByEventConstructor] readonly attribute EventTarget?     relatedTarget;
-    [MeasureAs=PrefixedMouseEventMovementX] readonly attribute long webkitMovementX;
-    [MeasureAs=PrefixedMouseEventMovementY] readonly attribute long webkitMovementY;
+    [MeasureAs=MouseEventMovementX] readonly attribute long             movementX;
+    [MeasureAs=MouseEventMovementY] readonly attribute long             movementY;
+    [MeasureAs=PrefixedMouseEventMovementX, ImplementedAs=movementX] readonly attribute long webkitMovementX;
+    [MeasureAs=PrefixedMouseEventMovementY, ImplementedAs=movementY] readonly attribute long webkitMovementY;
 
      void initMouseEvent([Default=Undefined] optional DOMString type,
                                        [Default=Undefined] optional boolean canBubble,