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..143eceedbad12260d8759a09856c95aac84103ad 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 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();
 }