LibWeb: Process session history queue after initial iframe navigation

This is not in the spec, but we need to make sure that "apply the
history step" for initial navigation to about:blank in iframe is
applied before subsequent navigations. Otherwise, "set ongoing
navigation" call during "about:blank" traversal might abort subsequent
ongoing navigation which is not expected to happen.

Userland/Libraries/LibWeb/HTML/TraversableNavigable.h

@@ -55,6 +55,11 @@ public:
         m_session_history_traversal_queue.append(move(steps));
     }
 
+    void process_session_history_traversal_queue()
+    {
+        m_session_history_traversal_queue.process();
+    }
+
     Page* page() { return m_page; }
     Page const* page() const { return m_page; }