Use VisibilityState enum and move WebPageVisibilityState to public/platform.

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 334 matching lines @@
     void setXMLEncoding(const String& encoding) { m_xmlEncoding = encoding; } // read-only property, only to be set from XMLDocumentParser
     void setXMLVersion(const String&, ExceptionState&);
     void setXMLStandalone(bool, ExceptionState&);
     void setHasXMLDeclaration(bool hasXMLDeclaration) { m_hasXMLDeclaration = hasXMLDeclaration ? 1 : 0; }
 
     virtual KURL baseURI() const override final;
 
     String origin() const { return securityOrigin()->toString(); }
 
     String visibilityState() const;
+    PageVisibilityState pageVisibilityState() const;
     bool hidden() const;
     void didChangeVisibilityState();
 
     PassRefPtrWillBeRawPtr<Node> adoptNode(PassRefPtrWillBeRawPtr<Node> source, ExceptionState&);
 
     PassRefPtrWillBeRawPtr<HTMLCollection> images();
     PassRefPtrWillBeRawPtr<HTMLCollection> embeds();
     PassRefPtrWillBeRawPtr<HTMLCollection> applets();
     PassRefPtrWillBeRawPtr<HTMLCollection> links();
     PassRefPtrWillBeRawPtr<HTMLCollection> forms();
@@ skipping 768 matching lines @@
 
     void updateTitle(const String&);
     void updateFocusAppearanceTimerFired(Timer<Document>*);
     void updateBaseURL();
 
     void executeScriptsWaitingForResourcesTimerFired(Timer<Document>*);
 
     void loadEventDelayTimerFired(Timer<Document>*);
     void pluginLoadingTimerFired(Timer<Document>*);
 
-    PageVisibilityState pageVisibilityState() const;
-
     // Note that dispatching a window load event may cause the LocalDOMWindow to be detached from
     // the LocalFrame, so callers should take a reference to the LocalDOMWindow (which owns us) to
     // prevent the Document from getting blown away from underneath them.
     void dispatchWindowLoadEvent();
 
     void addListenerType(ListenerType listenerType) { m_listenerTypes |= listenerType; }
     void addMutationEventListenerTypeIfEnabled(ListenerType);
 
     void didAssociateFormControlsTimerFired(Timer<Document>*);
 
@@ skipping 275 matching lines @@
 DEFINE_TYPE_CASTS(TreeScope, Document, document, true, true);
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLiveDocumentInstances();
 #endif
 
 #endif // Document_h