Bug 1133201 - part 1 - treat null links in BrowserUtils.linkHasNoRefe…

…rrer as specifying rel="noreferrer"; r=Gijs,mconley

It turns out that treating null links as being OK for passing along
referrer information means that we now pass referrer information for
plain text "links" that are opened via the context menu.  For referrer
information, we should take a much more conservative approach, and
declare that null links are always treated as if they had
rel="noreferrer".

toolkit/modules/BrowserUtils.jsm

@@ -217,8 +217,11 @@ this.BrowserUtils = {
    * @return a boolean indicating if linkNode has a rel="noreferrer" attribute.
    */
   linkHasNoReferrer: function (linkNode) {
+    // A null linkNode typically means that we're checking a link that wasn't
+    // provided via an <a> link, like a text-selected URL.  Don't leak
+    // referrer information in this case.
     if (!linkNode)
-      return false;
+      return true;
 
     let rel = linkNode.getAttribute("rel");
     if (!rel)