[iOS] Only GoBack if CanGoBack on committed interstitials

Check whether it's possible to go back before trying to perform a
back navigation on a committed interstitial when the "Back to safety"
button is pressed. This CL fixes the stability issue--a follow-up CL
will fix the actual functionality of the button on e.g. interstitial
opened in new tab.

Bug: 1059142
Change-Id: Icc03af381ca376731b451238ccb671a216764bfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2111411Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Livvie Lin <livvielin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751881}

ios/chrome/browser/interstitials/ios_chrome_controller_client.mm

@@ -41,7 +41,9 @@ void IOSChromeControllerClient::LaunchDateAndTimeSettings() {
 }
 
 void IOSChromeControllerClient::GoBack() {
-  web_state_->GetNavigationManager()->GoBack();
+  if (CanGoBack()) {
+    web_state_->GetNavigationManager()->GoBack();
+  }
 }
 
 bool IOSChromeControllerClient::CanGoBack() {