Navigation transitions: Block first response until after transitions have run.

content/public/browser/web_contents_observer.h

Index: content/public/browser/web_contents_observer.h
diff --git a/content/public/browser/web_contents_observer.h b/content/public/browser/web_contents_observer.h
index da9e8239d688412d1bc617f5e35d01c2e6b55b77..f39e56a37c96517a77339d946d6b03dfa430a5dd 100644
--- a/content/public/browser/web_contents_observer.h
+++ b/content/public/browser/web_contents_observer.h
@@ -246,6 +246,10 @@ class CONTENT_EXPORT WebContentsObserver : public IPC::Listener,
   virtual void DidStartLoading(RenderViewHost* render_view_host) {}
   virtual void DidStopLoading(RenderViewHost* render_view_host) {}
 
+  // This method is invoked when the request is deferred immediately after
+  // receiving response headers.
+  virtual void DidDeferAfterResponseStarted() {}
+
   // When WebContents::Stop() is called, the WebContents stops loading and then
   // invokes this method. If there are ongoing navigations, their respective
   // failure methods will also be invoked.