Adding a special build and test script for athena

This will land shortly before 650403004 gets landed.

BUG=410400
TEST=-

Review URL: https://codereview.chromium.org/652293005

Cr-Commit-Position: refs/heads/master@{#301122}

testing/buildbot/chromium_athena.json

+{
+  "compile_targets": [
+    "chrome",
+    "athena_main"
+  ],
+  "gtest_tests": [
+    {
+      "test": "browser_tests",
+      "swarming": {
+        "can_use_on_swarming_builders": true,
+        "shards": 5
+      }
+    },
+    {
+      "test": "athena_unittests",
+      "swarming": {
+        "can_use_on_swarming_builders": true,
+        "shards": 5
+      }
+    }
+  ]
+}