Use VisibilityState enum and move WebPageVisibilityState to public/platform.

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index deb721a5197fba62dd49aeba883f782de5281b3e..866d2bbc668a366f16a55461a80d5b2d32d05d50 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -352,6 +352,7 @@ public:
     String origin() const { return securityOrigin()->toString(); }
 
     String visibilityState() const;
+    PageVisibilityState pageVisibilityState() const;
     bool hidden() const;
     void didChangeVisibilityState();
 
@@ -1140,8 +1141,6 @@ private:
     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.