Added arm test spec.

Will be used by chromium_trybot_arm recipe.

R=mseaborn@chromium.org, phajdan.jr@chromium.org
BUG=359338

Review URL: https://codereview.chromium.org/246623009

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@269655 4ff67af0-8c30-449e-8e8b-ad334ec8d88c