Use VisibilityState enum and move WebPageVisibilityState to public/platform.

Source/modules/serviceworkers/ServiceWorkerWindowClient.h

 // Copyright 2014 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 ServiceWorkerWindowClient_h
 #define ServiceWorkerWindowClient_h
 
 #include "bindings/core/v8/ScriptPromise.h"
 #include "modules/serviceworkers/ServiceWorkerClient.h"
 #include "platform/heap/Handle.h"
 #include "wtf/Forward.h"
 
 namespace blink {
 
 class ScriptState;
 
 class ServiceWorkerWindowClient final : public ServiceWorkerClient {
     DEFINE_WRAPPERTYPEINFO();
 public:
     static ServiceWorkerWindowClient* create(const WebServiceWorkerClientInfo&);
     ~ServiceWorkerWindowClient() override;
 
     // WindowClient.idl
-    String visibilityState() const { return m_visibilityState; }
+    String visibilityState() const;
     bool focused() const { return m_isFocused; }
     String frameType() const;
     ScriptPromise focus(ScriptState*);
 
     virtual void trace(Visitor*) override;
 
 private:
     explicit ServiceWorkerWindowClient(const WebServiceWorkerClientInfo&);
 
+    // FIXME: remove m_visibilityState when m_pageVisibilityState will be used
+    // by Chromium.
     String m_visibilityState;
+    WebPageVisibilityState m_pageVisibilityState;
     bool m_isFocused;
     WebURLRequest::FrameType m_frameType;
 };
 
 } // namespace blink
 
 #endif // ServiceWorkerWindowClient_h