Mojo: Run mojo_shell_tests on trybots on all platforms.

R=sky@chromium.org

Review URL: https://codereview.chromium.org/414143002

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@285474 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
Adding new tryserver.chromium.* json files to allow the 'chromium' recipe to run on the tryserver.

R=iannucci@chromium.org
BUG=395196

Review URL: https://codereview.chromium.org/413223003

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@285751 4ff67af0-8c30-449e-8e8b-ad334ec8d88c