Support interstitials with no primary button

ios/chrome/browser/interstitials/ios_chrome_controller_client.mm

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ios/chrome/browser/interstitials/ios_chrome_controller_client.h"
 
 #include "base/logging.h"
 #include "components/security_interstitials/core/metrics_helper.h"
 #include "ios/chrome/browser/application_context.h"
 #include "ios/chrome/browser/browser_state/chrome_browser_state.h"
@@ skipping 27 matching lines @@
 
 void IOSChromeControllerClient::LaunchDateAndTimeSettings() {
   NOTREACHED();
 }
 
 void IOSChromeControllerClient::GoBack() {
   DCHECK(web_interstitial_);
   web_interstitial_->DontProceed();
 }
 
+bool IOSChromeControllerClient::CanGoBack() {
+  return true;

[rohitrao (ping after 24h), 2017/05/09 10:38:40]

Is this meant to always be true, even if the underlying WebState cannot go back?
 The //content implementation delegates to WebContents -- perhaps it's worth
having some documentation explaining why iOS behaves differently?

[edwardjung, 2017/05/09 11:20:25]

This was added primarily for webview but you're right it should be a check here,
is there an equivalent check in the WebState for this?

[edwardjung, 2017/05/09 11:47:47]

I found the equivalent and implemented here. 

+}
+
 void IOSChromeControllerClient::GoBackAfterNavigationCommitted() {
   NOTREACHED();
 }
 
 void IOSChromeControllerClient::Proceed() {
   DCHECK(web_interstitial_);
   web_interstitial_->Proceed();
 }
 
 void IOSChromeControllerClient::Reload() {
@@ skipping 14 matching lines @@
 PrefService* IOSChromeControllerClient::GetPrefService() {
   return ios::ChromeBrowserState::FromBrowserState(
              web_state_->GetBrowserState())
       ->GetPrefs();
 }
 
 const std::string IOSChromeControllerClient::GetExtendedReportingPrefName()
     const {
   return std::string();
 }