Mojo: Enable mojo_shell_tests on Mac (on the bots).

testing/buildbot/chromium.mac.json

Index: testing/buildbot/chromium.mac.json
diff --git a/testing/buildbot/chromium.mac.json b/testing/buildbot/chromium.mac.json
index 16a887d7f7256d793282cc65522065ad80d72ad4..b36baa5872617c66d6d7262f88b779a1098a29bf 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -48,6 +48,7 @@
       "mojo_public_system_unittests",
       "mojo_public_utility_unittests",
       "mojo_service_manager_unittests",
+      "mojo_shell_tests",
       "mojo_system_unittests",
       "sandbox_mac_unittests",
       {"test": "browser_tests", "shard_index": 2, "total_shards": 3}
@@ -102,6 +103,7 @@
       "mojo_public_system_unittests",
       "mojo_public_utility_unittests",
       "mojo_service_manager_unittests",
+      "mojo_shell_tests",
       "mojo_system_unittests",
       "sandbox_mac_unittests",
       {"test": "browser_tests", "shard_index": 2, "total_shards": 3}
@@ -161,6 +163,7 @@
       "mojo_public_system_unittests",
       "mojo_public_utility_unittests",
       "mojo_service_manager_unittests",
+      "mojo_shell_tests",
       "mojo_system_unittests",
       {"test": "browser_tests", "shard_index": 3, "total_shards": 4}
     ]
@@ -219,6 +222,7 @@
       "mojo_public_system_unittests",
       "mojo_public_utility_unittests",
       "mojo_service_manager_unittests",
+      "mojo_shell_tests",
       "mojo_system_unittests",
       {"test": "browser_tests", "shard_index": 3, "total_shards": 4}
     ]