Mojo: Silence death ...

... test thread warnings (on Linux). Actually, just run them in a
thread-safe way (even though we shouldn't have any actual thread safety
issues).

R=darin@chromium.org

Review URL: https://codereview.chromium.org/185873002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254525 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254525