Support interstitials with no primary button

ios/chrome/browser/interstitials/ios_chrome_controller_client.mm

Index: ios/chrome/browser/interstitials/ios_chrome_controller_client.mm
diff --git a/ios/chrome/browser/interstitials/ios_chrome_controller_client.mm b/ios/chrome/browser/interstitials/ios_chrome_controller_client.mm
index fa640eae5a9a29b6f868782bb1ddded14614fb95..1df9dc058b2954d5f79926220a43d6cdcb13d771 100644
--- a/ios/chrome/browser/interstitials/ios_chrome_controller_client.mm
+++ b/ios/chrome/browser/interstitials/ios_chrome_controller_client.mm
@@ -45,6 +45,10 @@ void IOSChromeControllerClient::GoBack() {
   web_interstitial_->DontProceed();
 }
 
+bool IOSChromeControllerClient::CanGoBack() {
+  return web_state_->GetNavigationManager()->CanGoBack();
+}
+
 void IOSChromeControllerClient::GoBackAfterNavigationCommitted() {
   NOTREACHED();
 }