Balance browser_tests across all chromiumos dbg test builders

R=jamescook@chromium.org, phajdan.jr@chromium.org
BUG=366362

Review URL: https://codereview.chromium.org/254443003

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@273318 4ff67af0-8c30-449e-8e8b-ad334ec8d88c