Implement ComputedStylePropertyMap for Typed OM.

third_party/WebKit/Source/core/css/cssom/ImmutableStylePropertyMap.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 ImmutableStylePropertyMap_h
 #define ImmutableStylePropertyMap_h
 
 #include "core/css/cssom/StylePropertyMap.h"
 
 namespace blink {
 
 class CORE_EXPORT ImmutableStylePropertyMap : public StylePropertyMap {
     WTF_MAKE_NONCOPYABLE(ImmutableStylePropertyMap);
 public:
     void set(CSSPropertyID, StyleValueOrStyleValueSequenceOrString&, ExceptionState& exceptionState) override
     {
         exceptionState.throwTypeError("This StylePropertyMap is immutable.");
     }
 
     void append(CSSPropertyID, StyleValueOrStyleValueSequenceOrString&, ExceptionState& exceptionState) override
     {
         exceptionState.throwTypeError("This StylePropertyMap is immutable.");
     }
 
     void remove(CSSPropertyID, ExceptionState& exceptionState) override
     {
         exceptionState.throwTypeError("This StylePropertyMap is immutable.");
     }
+
+protected:
+    ImmutableStylePropertyMap()
+        : StylePropertyMap() { }
 };
 
 } // namespace blink
 
 #endif