Disable mojo tests on chromium_trybot, they are broken on Mac

TBR=darin

BUG=346766, 332239

Review URL: https://codereview.chromium.org/180193002

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@253210 4ff67af0-8c30-449e-8e8b-ad334ec8d88c