[CSS Typed OM] Rearrange StylePropertyMap classes to match the new spec.

third_party/WebKit/Source/core/css/cssom/WindowGetComputedStyle.h

 // Copyright 2016 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 WindowGetComputedStyle_h
 #define WindowGetComputedStyle_h
 
 #include "core/css/CSSComputedStyleDeclaration.h"
 #include "core/css/cssom/ComputedStylePropertyMap.h"
 
 namespace blink {
 
 class WindowGetComputedStyle {
   STATIC_ONLY(WindowGetComputedStyle);
 
  public:
-  static StylePropertyMap* getComputedStyleMap(const LocalDOMWindow&,
-                                               Element* element,
-                                               const String& pseudoElement) {
+  static StylePropertyMapReadonly* getComputedStyleMap(
+      const LocalDOMWindow&,
+      Element* element,
+      const String& pseudoElement) {
     DCHECK(element);
     return ComputedStylePropertyMap::create(element, pseudoElement);
   }
 };
 
 }  // namespace blink
 
 #endif  // WindowGetComputedStyle_h