Add UseCounters for non-standard MouseEvent IDL attributes

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 f5b03c862d9ef20bacdf38f6dd2d5f178741c9c3..feda914c789cd6cf7c5d304f79033e9cfe433d9e 100644
--- a/Source/core/events/MouseEvent.idl
+++ b/Source/core/events/MouseEvent.idl
@@ -51,14 +51,14 @@
                                        [Default=Undefined] optional unsigned short button,
                                        [Default=Undefined] optional EventTarget relatedTarget);
 
-    // extensions
-    readonly attribute long             offsetX;
-    readonly attribute long             offsetY;
-    readonly attribute long             x;
-    readonly attribute long             y;
-    readonly attribute Node             fromElement;
-    readonly attribute Node             toElement;
+    // Non-standard
+    [MeasureAs=MouseEventOffsetX] readonly attribute long offsetX;
+    [MeasureAs=MouseEventOffsetY] readonly attribute long offsetY;
+    [MeasureAs=MouseEventX] readonly attribute long x;
+    [MeasureAs=MouseEventY] readonly attribute long y;
+    [MeasureAs=MouseEventFromElement] readonly attribute Node fromElement;
+    [MeasureAs=MouseEventToElement] readonly attribute Node toElement;
 
-    readonly attribute DataTransfer     dataTransfer;
+    // FIXME: this belongs on the DragEvent interface
+    readonly attribute DataTransfer dataTransfer;
 };
-