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

content/public/test/test_navigation_observer.cc

Index: content/public/test/test_navigation_observer.cc
diff --git a/content/public/test/test_navigation_observer.cc b/content/public/test/test_navigation_observer.cc
index e2c69b74369fabd0f3de5c48c48634464cd39067..cad43a8cb6348ba6ff10cac34cfbff4c3f10d6c0 100644
--- a/content/public/test/test_navigation_observer.cc
+++ b/content/public/test/test_navigation_observer.cc
@@ -30,6 +30,10 @@ class TestNavigationObserver::TestWebContentsObserver
     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);
   }
@@ -118,6 +122,13 @@ void TestNavigationObserver::OnNavigationEntryCommitted(
   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;
 }