Bug 1403733 - Fix single button styling on Flash doorhanger r=Felipe

There's not a great selector that covers the case of either button
being hidden, so the best option seems to be an additional
attribute.

MozReview-Commit-ID: CMdjWQnPjGA

--HG--
extra : rebase_source : ad6dbd1d9a85cbd70375013687330eb13135caa7

browser/base/content/urlbarBindings.xml

@@ -2886,6 +2886,12 @@ file, You can obtain one at http://mozilla.org/MPL/2.0/.
           } else if (button2.default) {
             this._secondaryButton.setAttribute("default", "true");
           }
+
+          if (this._primaryButton.hidden) {
+            this._secondaryButton.setAttribute("alone", "true");
+          } else if (this._secondaryButton.hidden) {
+            this._primaryButton.setAttribute("alone", "true");
+          }
         ]]></body>
       </method>
       <method name="_setupDescription">

toolkit/themes/shared/popupnotification.inc.css

@@ -78,7 +78,7 @@
   color: graytext;
 }
 
-.popup-notification-button[default] {
+.popup-notification-button[default]:not([alone]) {
   flex: 0 50%;
 }