"Go back" on the block interstitial closes the tab if there is no previous URL.

content/public/test/test_navigation_observer.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_PUBLIC_TEST_TEST_NAVIGATION_OBSERVER_H_
 #define CONTENT_PUBLIC_TEST_TEST_NAVIGATION_OBSERVER_H_
 
 #include <set>
 
 #include "base/callback.h"
@@ skipping 34 matching lines @@
   class TestWebContentsObserver;
 
   // Callbacks for WebContents-related events.
   void OnWebContentsCreated(WebContents* web_contents);
   void OnWebContentsDestroyed(TestWebContentsObserver* observer,
                               WebContents* web_contents);
   void OnNavigationEntryCommitted(
       TestWebContentsObserver* observer,
       WebContents* web_contents,
       const LoadCommittedDetails& load_details);
+  void OnDidAttachInterstitialPage(WebContents* web_contents);
   void OnDidStartLoading(WebContents* web_contents);
   void OnDidStopLoading(WebContents* web_contents);
 
   // If true the navigation has started.
   bool navigation_started_;
 
   // The number of navigations that have been completed.
   int navigations_completed_;
 
   // The number of navigations to wait for.
   int number_of_navigations_;
 
   // The MessageLoopRunner used to spin the message loop.
   scoped_refptr<MessageLoopRunner> message_loop_runner_;
 
   // Callback invoked on WebContents creation.
   base::Callback<void(WebContents*)> web_contents_created_callback_;
 
   // Living TestWebContentsObservers created by this observer.
   std::set<TestWebContentsObserver*> web_contents_observers_;
 
   DISALLOW_COPY_AND_ASSIGN(TestNavigationObserver);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_TEST_TEST_NAVIGATION_OBSERVER_H_