Buildbot: Add ipc_mojo_unittests to Linux bots.

Linux only. Mac and Windows isn't ready.

TEST=none
BUG=none
R=phajdan.jr@chromium.org

Review URL: https://codereview.chromium.org/452513002

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@288824 4ff67af0-8c30-449e-8e8b-ad334ec8d88c