Make ClientHintsPreferences class work like a class.

Source/core/dom/Document.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 1004 matching lines @@
 
     v8::Local<v8::Object> wrap(v8::Isolate*, v8::Local<v8::Object> creationContext) override;
     v8::Local<v8::Object> associateWithWrapper(v8::Isolate*, const WrapperTypeInfo*, v8::Local<v8::Object> wrapper) override WARN_UNUSED_RETURN;
 
     OriginsUsingFeatures::Value& originsUsingFeaturesValue() { return m_originsUsingFeaturesValue; }
 
     NthIndexCache* nthIndexCache() const { return m_nthIndexCache; }
 
     bool isPrivilegedContext(String& errorMessage, const PrivilegeContextCheck = StandardPrivilegeCheck) const override;
 
-    void setClientHintsPreferences(const ClientHintsPreferences& preferences) { m_clientHintsPreferences.set(preferences); }
-    const ClientHintsPreferences& clientHintsPreferences() const { return m_clientHintsPreferences; }
+    ClientHintsPreferences& clientHintsPreferences() { return m_clientHintsPreferences; }
 
     CanvasFontCache* canvasFontCache();
 
     void incrementNodeCount() { m_nodeCount++; }
     void decrementNodeCount()
     {
         ASSERT(m_nodeCount > 0);
         m_nodeCount--;
     }
     int nodeCount() const { return m_nodeCount; }
@@ skipping 384 matching lines @@
 DEFINE_TYPE_CASTS(TreeScope, Document, document, true, true);
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 CORE_EXPORT void showLiveDocumentInstances();
 #endif
 
 #endif // Document_h