Fix possible map::end() dereference in AppCacheUpdateJob triggered by a compromised renderer.

BUG=551044

Review URL: https://codereview.chromium.org/1418783005



Change-Id: Ie3f90de5a5d6e85767a49ac7d8b8f05a8c6335f8
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>