Add perf_dashboard_machine_group property to chromium.webrtc bots

This seems to be the only way to pass this property to runtest.py if we're not on Buildbot:
https://cs.chromium.org/chromium/build/scripts/slave/recipe_modules/chromium/api.py?rcl=0547a2d019f638868abffa0bd525d9e0ea9631d1&l=709
https://cs.chromium.org/chromium/build/scripts/slave/runtest.py?l=367&rcl=ccdd71cae2e6bce819a6d8f8b1dcaa27a9e31141

TBR: jbudorick@chromium.org
Bug: 877018
Change-Id: Icadb0abfd036a90735a6b194a0de1958e49107f3
Reviewed-on: https://chromium-review.googlesource.com/c/1338185
Commit-Queue: Oleh Prypin <oprypin@chromium.org>
Reviewed-by: Patrik Höglund <phoglund@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608689}

infra/config/global/cr-buildbucket.cfg

@@ -3397,6 +3397,7 @@ buckets {
         cipd_package: "infra/recipe_bundles/chromium.googlesource.com/chromium/tools/build"
         cipd_version: "refs/heads/master"
         properties: "mastername:chromium.webrtc"
+        properties: "perf_dashboard_machine_group:ChromiumWebRTC"
         name: "chromium"
       }
       service_account: "chromium-ci-builder@chops-service-accounts.iam.gserviceaccount.com"