Don't use the previous URL if current virtual URL is safebrowsing

Fixes #10143

Auditors: @diracdeltas, @alexwykoff

Test Plan:
1. open new tab
2. navigate to downloadme.org
3. notice that URL is properly "about:safebrowsing#downloadme.org"

app/browser/tabs.js

@@ -302,7 +302,8 @@ const fixDisplayURL = (navigationEntry, controller) => {
     navigationEntry.virtualURL = getSourceAboutUrl(navigationEntry.virtualURL)
   }
 
-  if (isIntermediateAboutPage(navigationEntry.virtualURL)) {
+  if (isIntermediateAboutPage(navigationEntry.virtualURL) &&
+    !navigationEntry.virtualURL.startsWith('about:safebrowsing#')) {
     const previousEntry = controller.getEntryAtOffset(-1)
     if (!controller.canGoForward() && previousEntry) {
       navigationEntry.virtualURL = previousEntry.virtualURL