Use VisibilityState enum and move WebPageVisibilityState to public/platform.

Source/core/dom/Document.idl

 /*
  * Copyright (C) 2006, 2007, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006, 2007 Samuel Weinig <sam@webkit.org>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  */
 
 callback CustomElementConstructor = Element ();
 
 typedef (CanvasRenderingContext2D or WebGLRenderingContext) RenderingContext;
 
+// http://www.w3.org/TR/page-visibility/#VisibilityState
+enum VisibilityState { "hidden", "visible", "prerender", "unloaded" };
+
 interface Document : Node {
 
     // DOM Level 1 Core
     readonly attribute DocumentType doctype;
     readonly attribute DOMImplementation implementation;
     readonly attribute Element documentElement;
 
     [CustomElementCallbacks, PerWorldBindings, RaisesException] Element createElement(DOMString tagName);
     DocumentFragment createDocumentFragment();
     Text createTextNode(DOMString data);
@@ skipping 154 matching lines @@
                                              [Default=Undefined] optional double webkitRadiusY,
                                              [Default=Undefined] optional float webkitRotationAngle,
                                              [Default=Undefined] optional float webkitForce);
     [RuntimeEnabled=Touch] TouchList createTouchList(Touch... touches);
 
     [CallWith=ScriptState, CustomElementCallbacks, RaisesException, MeasureAs=DocumentRegisterElement] CustomElementConstructor registerElement(DOMString name, optional ElementRegistrationOptions options);
     [CustomElementCallbacks, PerWorldBindings, RaisesException] Element createElement(DOMString localName, DOMString? typeExtension);
     [CustomElementCallbacks, RaisesException] Element createElementNS(DOMString? namespaceURI, DOMString qualifiedName, DOMString? typeExtension);
 
     // Page visibility API.
-    readonly attribute DOMString visibilityState;
+    readonly attribute VisibilityState visibilityState;
     readonly attribute boolean hidden;
 
     // Deprecated prefixed page visibility API.
     // TODO(davidben): This is a property so attaching a deprecation warning results in false positives when outputting
     // document in the console. It's possible http://crbug.com/43394 will resolve this.
     [MeasureAs=PrefixedPageVisibility, ImplementedAs=visibilityState] readonly attribute DOMString webkitVisibilityState;
     [MeasureAs=PrefixedPageVisibility, ImplementedAs=hidden] readonly attribute boolean webkitHidden;
 
     readonly attribute HTMLScriptElement currentScript;
 };
 
 Document implements GlobalEventHandlers;
 Document implements ParentNode;