[resultdb] enable resultdb for junit_tests on try builders at 30%

Change-Id: Ifeb9bef8dae18092e105085b1497859bd67865c9
Bug: 1084332
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2589042Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Scott Lee <ddoman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836702}

infra/config/lib/try.star

@@ -157,6 +157,7 @@ def try_builder(
     # Enable "chromium.resultdb.result_sink" on try builders.
     experiments = experiments or {}
     experiments.setdefault("chromium.resultdb.result_sink", 100)
+    experiments.setdefault("chromium.resultdb.result_sink.junit_tests", 30)
 
     merged_resultdb_bigquery_exports = [
         resultdb.export_test_results(