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

chrome/browser/managed_mode/managed_mode_interstitial.cc

Index: chrome/browser/managed_mode/managed_mode_interstitial.cc
diff --git a/chrome/browser/managed_mode/managed_mode_interstitial.cc b/chrome/browser/managed_mode/managed_mode_interstitial.cc
index 225b776e339cd7856a0018852bf69b555f42cedf..9d97ccc40fce4fe04b0c5e13a5801e0c367f0b5b 100644
--- a/chrome/browser/managed_mode/managed_mode_interstitial.cc
+++ b/chrome/browser/managed_mode/managed_mode_interstitial.cc
@@ -224,6 +224,11 @@ void ManagedModeInterstitial::OnFilteringPrefsChanged() {
 }
 
 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));
 }