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

content/public/test/test_navigation_observer.cc

 // 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.
 
 #include "content/public/test/test_navigation_observer.h"
 
 #include "base/bind.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/stl_util.h"
@@ skipping 12 matching lines @@
         parent_(parent) {
   }
 
  private:
   // WebContentsObserver:
   virtual void NavigationEntryCommitted(
       const LoadCommittedDetails& load_details) OVERRIDE {
     parent_->OnNavigationEntryCommitted(this, web_contents(), load_details);
   }
 
+  virtual void DidAttachInterstitialPage() OVERRIDE {
+    parent_->OnDidAttachInterstitialPage(web_contents());
+  }
+
   virtual void WebContentsDestroyed(WebContents* web_contents) OVERRIDE {
     parent_->OnWebContentsDestroyed(this, web_contents);
   }
 
   virtual void DidStartLoading(RenderViewHost* render_view_host) OVERRIDE {
     parent_->OnDidStartLoading(web_contents());
   }
 
   virtual void DidStopLoading(RenderViewHost* render_view_host) OVERRIDE {
     parent_->OnDidStopLoading(web_contents());
@@ skipping 68 matching lines @@
   delete observer;
 }
 
 void TestNavigationObserver::OnNavigationEntryCommitted(
     TestWebContentsObserver* observer,
     WebContents* web_contents,
     const LoadCommittedDetails& load_details) {
   navigation_started_ = true;
 }
 
+void TestNavigationObserver::OnDidAttachInterstitialPage(
+    WebContents* web_contents) {
+  // Going to an interstitial page does not trigger NavigationEntryCommitted,
+  // but has the same meaning for us here.
+  navigation_started_ = true;
+}
+
 void TestNavigationObserver::OnDidStartLoading(WebContents* web_contents) {
   navigation_started_ = true;
 }
 
 void TestNavigationObserver::OnDidStopLoading(WebContents* web_contents) {
   if (!navigation_started_)
     return;
 
   ++navigations_completed_;
   if (navigations_completed_ == number_of_navigations_) {
     navigation_started_ = false;
     message_loop_runner_->Quit();
   }
 }
 
 }  // namespace content