Fix thread destruction ordering. Cache thread needs to outlive the IO thread so the IO thread can properly shut down. This was causing mojo_shell to hang at exit.

R=darin@chromium.org, abarth@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/77453010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236558 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236558