infra: Add builder group to CI properties. The builder group is necessary for recipe scripts. A first preliminary step towards standalone testers. Note on naming: Chromium bots use "chromium.X". WebRTC and V8 use "client.webrtc" etc. Going with just "angle" for simplicity. Bug: angleproject:5114 Change-Id: Ie0e47aa28eaeab4ebe8d13900c1b68cad8a7a1a5 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2805892 Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>