Sync the getComputedStyle() IDL with the spec

Source/core/frame/DOMWindow.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef DOMWindow_h
 #define DOMWindow_h
 
 #include "core/events/EventTarget.h"
 #include "core/frame/DOMWindowBase64.h"
 #include "platform/heap/Handle.h"
@@ skipping 139 matching lines @@
     void scroll(double x, double y, const ScrollOptions& scrollOptions, ExceptionState& exceptionState) const { scrollTo(x, y, scrollOptions, exceptionState); }
     virtual void moveBy(float x, float y) const = 0;
     virtual void moveTo(float x, float y) const = 0;
 
     virtual void resizeBy(float x, float y) const = 0;
     virtual void resizeTo(float width, float height) const = 0;
 
     virtual PassRefPtrWillBeRawPtr<MediaQueryList> matchMedia(const String&) = 0;
 
     // DOM Level 2 Style Interface
-    virtual PassRefPtrWillBeRawPtr<CSSStyleDeclaration> getComputedStyle(Element*, const String& pseudoElt) const = 0;
+    virtual PassRefPtrWillBeRawPtr<CSSStyleDeclaration> getComputedStyle(Element*, const String& pseudoElt = String()) const = 0;
 
     // WebKit extensions
     virtual PassRefPtrWillBeRawPtr<CSSRuleList> getMatchedCSSRules(Element*, const String& pseudoElt) const = 0;
 
     // WebKit animation extensions
     virtual int requestAnimationFrame(RequestAnimationFrameCallback*) = 0;
     virtual int webkitRequestAnimationFrame(RequestAnimationFrameCallback*) = 0;
     virtual void cancelAnimationFrame(int id) = 0;
 
     void captureEvents() { }
@@ skipping 23 matching lines @@
     DEFINE_ATTRIBUTE_EVENT_LISTENER(orientationchange);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(touchstart);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(touchmove);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(touchend);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(touchcancel);
 };
 
 } // namespace blink
 
 #endif // DOMWindow_h