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

chrome/browser/managed_mode/managed_mode_interstitial.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 "chrome/browser/managed_mode/managed_mode_interstitial.h"
 
 #include "base/i18n/rtl.h"
 #include "base/metrics/histogram.h"
 #include "base/prefs/pref_service.h"
 #include "base/strings/utf_string_conversions.h"
@@ skipping 206 matching lines @@
   return url_filter->GetFilteringBehaviorForURL(url_) !=
          ManagedModeURLFilter::BLOCK;
 }
 
 void ManagedModeInterstitial::OnFilteringPrefsChanged() {
   if (ShouldProceed())
     interstitial_page_->Proceed();
 }
 
 void ManagedModeInterstitial::DispatchContinueRequest(bool continue_request) {
+  // If there is no history entry to go back to, close the tab instead.
+  int nav_entry_count = web_contents_->GetController().GetEntryCount();
+  if (!continue_request && nav_entry_count == 0)
+    web_contents_->Close();
+
   BrowserThread::PostTask(
       BrowserThread::IO, FROM_HERE, base::Bind(callback_, continue_request));
 }