Linux sandbox tests: always stop threads

Always stop threads in an accountable way in order to not pollute
other tests.

TEST=sandbox_linux_unittests
NOTRY=true

Review URL: https://codereview.chromium.org/145123012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250148 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250148