Support interstitials with no primary button

components/security_interstitials/core/controller_client.h

Index: components/security_interstitials/core/controller_client.h
diff --git a/components/security_interstitials/core/controller_client.h b/components/security_interstitials/core/controller_client.h
index 33d9cece49438ed842586b792cf8a538509ad5f8..adc3ca9c2a0e2c72cd0791bee5ca0c2ada0dfa69 100644
--- a/components/security_interstitials/core/controller_client.h
+++ b/components/security_interstitials/core/controller_client.h
@@ -71,6 +71,8 @@ class ControllerClient {
   // Close the error and go back to the previous page. This applies to
   // situations where navigation is blocked before committing.
   virtual void GoBack() = 0;
+  // Whether it is possible to go 'Back to safety'. Specifially for WebViews.

[felt, 2017/05/07 14:00:39]

Nit: Suggestion, leave off the last sentence. (I think we might end up wanting
to use this elsewhere too to fix some bugs, there are other situations besides
WebView where you can't actually 'go back'.)

[edwardjung, 2017/05/07 16:11:59]

Done.

+  virtual bool CanGoBack() = 0;
 
   // If the offending entry has committed, go back or to a safe page without
   // closing the error page. This error page will be closed when the new page