Bug 1524545 Move raptor-unity-webgl-firefox to tier 3 and stop running on autoland r=Bebe,perftest-reviewers

Differential Revision: https://phabricator.services.mozilla.com/D61456

--HG--
extra : moz-landing-system : lando

taskcluster/ci/test/raptor.yml

@@ -403,6 +403,11 @@ raptor-unity-webgl-firefox:
     variants: ["fission"]
     try-name: raptor-unity-webgl-firefox
     treeherder-symbol: Rap(ugl)
+    run-on-projects:
+        by-test-platform:
+            windows10-aarch64/opt: ['try']
+            default: ['try', 'mozilla-central']
+    tier: 3
     mozharness:
         extra-options:
             - --test=raptor-unity-webgl