Convert mojo_public_utility_unittests to run exclusively on Swarming

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 65b18bb1cc971286ffcd2d40371960c88ae43b8a..a746890f2644a470a9656724cfc202a1012b59f7 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -210,6 +210,9 @@
         "test": "mojo_public_system_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_utility_unittests"
       },
       {
@@ -467,6 +470,9 @@
         "test": "mojo_public_system_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_utility_unittests"
       },
       {
@@ -733,6 +739,9 @@
         "test": "mojo_public_system_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_utility_unittests"
       },
       {
@@ -999,6 +1008,9 @@
         "test": "mojo_public_system_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_utility_unittests"
       },
       {
@@ -1266,6 +1278,9 @@
         "test": "mojo_public_system_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_utility_unittests"
       },
       {