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

content/public/test/test_navigation_observer.h

Index: content/public/test/test_navigation_observer.h
diff --git a/content/public/test/test_navigation_observer.h b/content/public/test/test_navigation_observer.h
index f6e5e41b71dfb8b5039d8a820c6b01a79d4e6a9f..438af744eb56c076d561664818243aa4be4aabe8 100644
--- a/content/public/test/test_navigation_observer.h
+++ b/content/public/test/test_navigation_observer.h
@@ -52,6 +52,7 @@ class TestNavigationObserver {
       TestWebContentsObserver* observer,
       WebContents* web_contents,
       const LoadCommittedDetails& load_details);
+  void OnDidAttachInterstitialPage(WebContents* web_contents);
   void OnDidStartLoading(WebContents* web_contents);
   void OnDidStopLoading(WebContents* web_contents);