Use VisibilityState enum and move WebPageVisibilityState to public/platform.

Source/modules/serviceworkers/ServiceWorkerWindowClient.cpp

Index: Source/modules/serviceworkers/ServiceWorkerWindowClient.cpp
diff --git a/Source/modules/serviceworkers/ServiceWorkerWindowClient.cpp b/Source/modules/serviceworkers/ServiceWorkerWindowClient.cpp
index 7ec61f6ff32a9318eab6a3f999aa8792d29a0174..e4987e5d74a753773b40e01291657df462d38dc5 100644
--- a/Source/modules/serviceworkers/ServiceWorkerWindowClient.cpp
+++ b/Source/modules/serviceworkers/ServiceWorkerWindowClient.cpp
@@ -7,6 +7,7 @@
 
 #include "bindings/core/v8/CallbackPromiseAdapter.h"
 #include "bindings/core/v8/ScriptPromiseResolver.h"
+#include "core/page/PageVisibilityState.h"
 #include "core/page/WindowFocusAllowedIndicator.h"
 #include "modules/serviceworkers/ServiceWorkerError.h"
 #include "modules/serviceworkers/ServiceWorkerGlobalScopeClient.h"
@@ -23,6 +24,7 @@ ServiceWorkerWindowClient* ServiceWorkerWindowClient::create(const WebServiceWor
 ServiceWorkerWindowClient::ServiceWorkerWindowClient(const WebServiceWorkerClientInfo& info)
     : ServiceWorkerClient(info)
     , m_visibilityState(info.visibilityState)
+    , m_pageVisibilityState(info.pageVisibilityState)
     , m_isFocused(info.isFocused)
     , m_frameType(info.frameType)
 {
@@ -32,6 +34,14 @@ ServiceWorkerWindowClient::~ServiceWorkerWindowClient()
 {
 }
 
+String ServiceWorkerWindowClient::visibilityState() const
+{
+    // FIXME: temporary until m_pageVisibilityState is used in Chromium.
+    if (m_pageVisibilityState == WebPageVisibilityStateLast)
+        return m_visibilityState;
+    return pageVisibilityStateString(static_cast<PageVisibilityState>(m_pageVisibilityState));
+}
+
 String ServiceWorkerWindowClient::frameType() const
 {
     DEFINE_STATIC_LOCAL(const String, auxiliary, ("auxiliary"));