LibWeb: Add a "page showing" flag to documents

This will be used to determine whether "pageshow" and "pagehide" events
are appropriate. We won't actually make use of it until we implement
more of history traversal and document unloading.

Userland/Libraries/LibWeb/DOM/Document.h

@@ -294,6 +294,9 @@ public:
         --m_number_of_things_delaying_the_load_event;
     }
 
+    bool page_showing() const { return m_page_showing; }
+    void set_page_showing(bool value) { m_page_showing = value; }
+
 private:
     explicit Document(const AK::URL&);
 
@@ -378,6 +381,9 @@ private:
     NonnullRefPtr<HTML::History> m_history;
 
     size_t m_number_of_things_delaying_the_load_event { 0 };
+
+    // https://html.spec.whatwg.org/#page-showing
+    bool m_page_showing { false };
 };
 
 }