Adding Athenas browser and unit tests which are required for running the tests on a buildbot

BUG=410400
TEST=-

Review URL: https://codereview.chromium.org/641903002

Cr-Commit-Position: refs/heads/master@{#299302}

testing/buildbot/chromium.fyi.json

@@ -283,6 +283,23 @@
       "browser_tests"
     ]
   },
+  "Linux ChromiumOS Athena Tests (1)": {
+    "gtest_tests": [
+      {
+        "test": "athena_unittests",
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        }
+      },
+      {
+        "test": "browser_tests",
+        "swarming": {
+          "can_use_on_swarming_builders": true,
+          "shards": 5
+        }
+      }
+    ]
+  },
   "Site Isolation Linux": {
     "gtest_tests": [
       { "test": "content_unittests", "args": ["--site-per-process"] },

testing/buildbot/chromium_trybot.json

@@ -205,7 +205,7 @@
     {
       "test": "athena_unittests",
       "platforms": ["linux"],
-      "chromium_configs": ["chromium_chromeos", "chromium_chromeos_clang"]
+      "chromium_configs": ["chromium_chromeos_athena"]
     },
     {
       "test": "display_unittests",