Revert "Remove unneeded sleep const in Cache."

The bookmark cache is not window specific, and when the first window
disappears, so too does its liberator object.

This reverts commit 4f0154de457464ac7ffa64a6e9a7076e8af8964d.

common/content/bookmarks.js

@@ -82,6 +82,7 @@ function Bookmarks() //{{{
     function Cache(name, store, serial)
     {
         const rootFolders = [bookmarksService.toolbarFolder, bookmarksService.bookmarksMenuFolder, bookmarksService.unfiledBookmarksFolder];
+        const sleep = liberator.sleep;
 
         let bookmarks = [];
         let self = this;
@@ -141,7 +142,7 @@ function Bookmarks() //{{{
             if (loading)
             {
                 while (loading)
-                    liberator.sleep(10);
+                    sleep(10);
                 return bookmarks;
             }