Add UseCounters for init*Event methods that aren't already measured

Source/core/events/MutationEvent.idl

Index: Source/core/events/MutationEvent.idl
diff --git a/Source/core/events/MutationEvent.idl b/Source/core/events/MutationEvent.idl
index 9fde40fd2d5b41b08805b8664b07483a543e3c55..03ef8302f2b3b05f75073b738e3f5d549ea81d91 100644
--- a/Source/core/events/MutationEvent.idl
+++ b/Source/core/events/MutationEvent.idl
@@ -31,12 +31,12 @@ interface MutationEvent : Event {
     readonly attribute DOMString      attrName;
     readonly attribute unsigned short attrChange;
     // TODO(philipj): None of the initMutationEvent() arguments should be optional.
-    void initMutationEvent([Default=Undefined] optional DOMString type,
-                                          [Default=Undefined] optional boolean bubbles,
-                                          [Default=Undefined] optional boolean cancelable,
-                                          [Default=Undefined] optional Node relatedNode,
-                                          [Default=Undefined] optional DOMString prevValue,
-                                          [Default=Undefined] optional DOMString newValue,
-                                          [Default=Undefined] optional DOMString attrName,
-                                          [Default=Undefined] optional unsigned short attrChange);
+    [Measure] void initMutationEvent([Default=Undefined] optional DOMString type,
+                                     [Default=Undefined] optional boolean bubbles,
+                                     [Default=Undefined] optional boolean cancelable,
+                                     [Default=Undefined] optional Node relatedNode,
+                                     [Default=Undefined] optional DOMString prevValue,
+                                     [Default=Undefined] optional DOMString newValue,
+                                     [Default=Undefined] optional DOMString attrName,
+                                     [Default=Undefined] optional unsigned short attrChange);
 };