Don't crash if DontProceed is chosen on the SSL interstitial page after Proceed is chosen.

chrome/browser/ssl/ssl_blocking_page.cc

Index: chrome/browser/ssl/ssl_blocking_page.cc
diff --git a/chrome/browser/ssl/ssl_blocking_page.cc b/chrome/browser/ssl/ssl_blocking_page.cc
index 2f65292db5364232a47ed1467cb90a933be9049b..79277029769918a9008074cf734b87732ee86294 100644
--- a/chrome/browser/ssl/ssl_blocking_page.cc
+++ b/chrome/browser/ssl/ssl_blocking_page.cc
@@ -146,7 +146,11 @@ void SSLBlockingPage::DontProceed() {
 }
 
 void SSLBlockingPage::NotifyDenyCertificate() {
-  DCHECK(callback_);
+  // It's possible that callback_ may not exist if the user clicks "Proceed"
+  // followed by pressing the back button before the interstitial is hidden.
+  // In that case the certificate will still be treated as allowed.
+  if (!callback_)
+    return;
 
   callback_->Run(handler_, false);
   delete callback_;