Make getComputedStyle throw for incorrect first parameter

Source/core/frame/LocalDOMWindow.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  *
  * 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 1366 matching lines @@
 
 StyleMedia* LocalDOMWindow::styleMedia() const
 {
     if (!m_media)
         m_media = StyleMedia::create(frame());
     return m_media.get();
 }
 
 PassRefPtrWillBeRawPtr<CSSStyleDeclaration> LocalDOMWindow::getComputedStyle(Element* elt, const String& pseudoElt) const
 {
-    if (!elt)
-        return nullptr;
-
+    ASSERT(elt);
     return CSSComputedStyleDeclaration::create(elt, false, pseudoElt);
 }
 
 PassRefPtrWillBeRawPtr<CSSRuleList> LocalDOMWindow::getMatchedCSSRules(Element* element, const String& pseudoElement) const
 {
     if (!element)
         return nullptr;
 
     if (!isCurrentlyDisplayedInFrame())
         return nullptr;
@@ skipping 617 matching lines @@
     return m_frameObserver->frame();
 }
 
 v8::Handle<v8::Object> LocalDOMWindow::wrap(v8::Handle<v8::Object> creationContext, v8::Isolate* isolate)
 {
     ASSERT_NOT_REACHED(); // LocalDOMWindow has [Custom=ToV8].
     return v8::Handle<v8::Object>();
 }
 
 } // namespace blink