Introduce ES6 iterator for DOM objects.

Source/core/testing/Internals.idl

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 [
     DoNotCheckConstants,
     WillBeGarbageCollected,
+    Iterable,
 ] interface Internals {
     DOMString address(Node node);
 
     GCObservation observeGC(any observed);
 
     [RaisesException, TypeChecking=Interface] DOMString elementRenderTreeAsText(Element element);
     boolean isPreloaded(DOMString url);
     boolean isLoadingFromMemoryCache(DOMString url);
 
     [TypeChecking=Interface] boolean isSharingStyle(Element element1, Element element2);
@@ skipping 233 matching lines @@
     // This function is for testing HitRegions on Canvas2D.
     unsigned long countHitRegions(CanvasRenderingContext2D context);
 
     DOMString serializeNavigationMarkup();
     void hideAllTransitionElements();
 
     [RaisesException] void forcePluginPlaceholder(HTMLElement plugin, DOMString htmlSource);
 
     DictionaryTest dictionaryTest();
 };