Add BUILD.gn for //testing/perf.

All hail GN. oRZ

BUG=none
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/380483002

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@282443 4ff67af0-8c30-449e-8e8b-ad334ec8d88c