ash: Add missing include to test from r236852.

Add a missing base/threading/sequenced_worker_pool.h include
to desktop_background_controller_unittest.cc.

BUG=none
TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/83153007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236854 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236854