[fenix] For https://github.com/mozilla-mobile/fenix/issues/20936: run perftests with conditioned profiles.

We've noticed that geckoview_example is ~300ms faster than fenix in cold page
load tests on arewefastyet for the Pixel 2. We suspect the main
difference is because geckoview_example runs with conditioned profiles
and fenix does not.

This PR is foremost an experiment to see if that's true because, after bug
1587542, we cannot run get results for fenix perftest PRs (i.e. it needs to be
merged into main). If we find that the results are not noisy, however, we
could end up leaving this in the tree. We've previously seen excessive
noise with fenix start up tests with conditioned profiles which is why
conditioned profiles are not currently enabled.

taskcluster/ci/browsertime/kind.yml

@@ -92,6 +92,7 @@ job-defaults:
             - '--browsertime-geckodriver=$MOZ_FETCHES_DIR/geckodriver'
             - '--browsertime-ffmpeg=$MOZ_FETCHES_DIR/ffmpeg-4.1.4-i686-static/bin/ffmpeg'
             - '--browsertime-browsertimejs=$MOZ_FETCHES_DIR/browsertime/node_modules/browsertime/bin/browsertime.js'
+            - '--conditioned-profile=settled'
     fetches:
         toolchain:
             - browsertime