Add helper method to check for invalid message source.

content/browser/web_contents/web_contents_impl.h

Index: content/browser/web_contents/web_contents_impl.h
diff --git a/content/browser/web_contents/web_contents_impl.h b/content/browser/web_contents/web_contents_impl.h
index 4248c356507270cd403ff40c7927e04839079109..d01eeeed56ca8f997f076b46db7c7fc0ee7074da 100644
--- a/content/browser/web_contents/web_contents_impl.h
+++ b/content/browser/web_contents/web_contents_impl.h
@@ -752,6 +752,10 @@ class CONTENT_EXPORT WebContentsImpl
                          RenderFrameHost* render_frame_host,
                          const IPC::Message& message);
 
+  // Checks whether render_frame_message_source_ is set to non-null value,
+  // otherwise it terminates the main frame renderer process.
+  bool HasValidFrameSource();
+
   // IPC message handlers.
   void OnThemeColorChanged(SkColor theme_color);
   void OnDidLoadResourceFromMemoryCache(const GURL& url,