Bug 957585 - Make jumplists listen for "active" idle topic instead of the defunct "back". r=jimm

browser/modules/WindowsJumpLists.jsm

@@ -573,7 +573,7 @@ this.WinTaskbarJumpList =
         }
       break;
 
-      case "back":
+      case "active":
         this._updateTimer();
       break;
     }