Ensure athena_unittests is run for chromeos tryjobs.

BUG=424093

Review URL: https://codereview.chromium.org/664543002

Cr-Commit-Position: refs/heads/master@{#299958}

testing/buildbot/chromium_trybot.json

@@ -218,6 +218,7 @@
       "test": "athena_unittests",
       "platforms": ["linux"],
       "chromium_configs": [
+        "chromium_chromeos",
         "chromium_chromeos_athena",
         "chromium_chromeos_ozone"
       ]