Add UseCounter for initMessageEvent (including prefixed version.)

Source/core/events/MessageEvent.idl

Index: Source/core/events/MessageEvent.idl
diff --git a/Source/core/events/MessageEvent.idl b/Source/core/events/MessageEvent.idl
index c482b082cfb950202291ce261805e40cfdb6ac74..ba8ad37731fc79b42c3ac49e62d09f1f66484d11 100644
--- a/Source/core/events/MessageEvent.idl
+++ b/Source/core/events/MessageEvent.idl
@@ -35,7 +35,7 @@
     [InitializedByEventConstructor] readonly attribute EventTarget? source; // May be a Window or a MessagePort
     [InitializedByEventConstructor, Custom=Getter] readonly attribute any data;
     [InitializedByEventConstructor] readonly attribute MessagePort[] ports;
-    [Custom] void initMessageEvent([Default=Undefined] optional DOMString typeArg,
+    [Custom, MeasureAs=InitMessageEvent] void initMessageEvent([Default=Undefined] optional DOMString typeArg,
                                    [Default=Undefined] optional boolean canBubbleArg,
                                    [Default=Undefined] optional boolean cancelableArg,
                                    [Default=Undefined] optional any dataArg,
@@ -44,7 +44,7 @@
                                    [Default=Undefined] optional Window sourceArg,
                                    [Default=Undefined] optional Array messagePorts);
 
-    [Custom] void webkitInitMessageEvent([Default=Undefined] optional DOMString typeArg,
+    [Custom, MeasureAs=PrefixedInitMessageEvent] void webkitInitMessageEvent([Default=Undefined] optional DOMString typeArg,
                                          [Default=Undefined] optional boolean canBubbleArg,
                                          [Default=Undefined] optional boolean cancelableArg,
                                          [Default=Undefined] optional any dataArg,