Avoid a redundant strlen

The GString knows perfectly well how long it is.  Pointed out in
https://bugzilla.gnome.org/show_bug.cgi?id=731158

gtk/gtkbookmarksmanager.c

@@ -160,7 +160,7 @@ save_bookmarks (GFile  *bookmarks_file,
     }
   if (!g_file_replace_contents (bookmarks_file,
 				contents->str,
-				strlen (contents->str),
+				contents->len,
 				NULL, FALSE, 0, NULL,
 				NULL, &error))
     goto out;