Introduce XMLHttpRequestEventTarget IDL interface

Source/core/xml/XMLHttpRequest.idl

Index: Source/core/xml/XMLHttpRequest.idl
diff --git a/Source/core/xml/XMLHttpRequest.idl b/Source/core/xml/XMLHttpRequest.idl
index 2c4f1203fe96610ab96f3c18c7872145007c8ab5..cecdfd0d89699755d9200e0858b82ecc9d36d0c3 100644
--- a/Source/core/xml/XMLHttpRequest.idl
+++ b/Source/core/xml/XMLHttpRequest.idl
@@ -42,17 +42,7 @@ enum XMLHttpRequestResponseType {
     Constructor,
     ConstructorCallWith=ScriptExecutionContext,
     CustomConstructor(optional XMLHttpRequestOptions options)
-] interface XMLHttpRequest : EventTarget {
-    // From XMLHttpRequestEventTarget
-    // event handler attributes
-    attribute EventHandler onabort;
-    attribute EventHandler onerror;
-    attribute EventHandler onload;
-    attribute EventHandler onloadend;
-    attribute EventHandler onloadstart;
-    attribute EventHandler onprogress;
-    attribute EventHandler ontimeout;
-
+] interface XMLHttpRequest : XMLHttpRequestEventTarget {
     // event handler attributes
     attribute EventHandler onreadystatechange;