Make getComputedStyle throw for incorrect first parameter

Source/core/frame/Window.idl

 /*
  * Copyright (C) 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 120 matching lines @@
     // DOM Level 2 AbstractView Interface
     readonly attribute Document document;
 
     // CSSOM View Module
     MediaQueryList matchMedia(DOMString query);
 
     // styleMedia has been removed from the CSSOM View specification.
     readonly attribute StyleMedia styleMedia;
 
     // DOM Level 2 Style Interface
-    CSSStyleDeclaration getComputedStyle([Default=Undefined] optional Element element,
-                                         [TreatUndefinedAs=NullString, Default=Undefined] optional DOMString? pseudoElement);
+    [TypeChecking=Interface] CSSStyleDeclaration getComputedStyle([Default=Undefined] optional Element element,
+                                                                 [TreatUndefinedAs=NullString, Default=Undefined] optional DOMString? pseudoElement);
 
     // WebKit extensions
     [MeasureAs=GetMatchedCSSRules] CSSRuleList getMatchedCSSRules([Default=Undefined] optional Element element,
                                                                   [TreatUndefinedAs=NullString, Default=Undefined] optional DOMString? pseudoElement);
 
     [Replaceable] readonly attribute double devicePixelRatio;
 
     [RuntimeEnabled=ApplicationCache, LogActivity=GetterOnly] readonly attribute ApplicationCache applicationCache;
 
     [RuntimeEnabled=SessionStorage, LogActivity=GetterOnly, RaisesException=Getter] readonly attribute Storage sessionStorage;
@@ skipping 64 matching lines @@
 // FIXME: make this typedef accurate once enough of http://crbug.com/240176
 // is in place.
 // FIXME: consider putting this typedef in an .idl file containing spec-wide
 // utility type definitions.
 typedef MessagePort Transferable;
 
 Window implements GlobalEventHandlers;
 Window implements WindowBase64;
 Window implements WindowEventHandlers;
 Window implements WindowTimers;