Sync the DOMStringMap interface with the spec

Source/core/dom/DOMStringMap.idl

 /*
  * Copyright (C) 2010 Apple Inc. All rights reserved.
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  * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
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+// https://html.spec.whatwg.org/#the-domstringmap-interface
+
 [
     OverrideBuiltins,
     SetWrapperReferenceFrom=element,
     WillBeGarbageCollected,
 ] interface DOMStringMap {
+    [ImplementedAs=item] getter DOMString (DOMString name);
+    [RaisesException] setter void (DOMString name, DOMString value);
+    // FIXME: The deleter return type should be void. crbug.com/462920
+    deleter boolean (DOMString name);
+
+    // FIXME: The indexed getter, setter and deleter are not in the
+    // spec and simply converts the index to a string.
     [NotEnumerable] getter DOMString (unsigned long index);
-    [RaisesException] setter DOMString (unsigned long index, DOMString value);
+    [RaisesException] setter void (unsigned long index, DOMString value);
     deleter boolean (unsigned long index);
-
-    [ImplementedAs=item] getter DOMString (DOMString name);
-    [RaisesException] setter DOMString (DOMString name, DOMString value);
-    deleter boolean (DOMString name);
 };