Enable swarming tests on trybots that supports them (*_chromium_rel_swarming).

All other builders ignore "swarming" section. Currently *_chromium_rel_swarming
can only be triggered manually.

R=phajdan.jr@chromium.org,maruel@chromium.org
BUG=354057

Review URL: https://codereview.chromium.org/334983005

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@277654 4ff67af0-8c30-449e-8e8b-ad334ec8d88c