Convert mojo_public_system_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 2327747bed4c1774aa967ba8b920eb883fa92b63..65b18bb1cc971286ffcd2d40371960c88ae43b8a 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -204,6 +204,9 @@
         "test": "mojo_public_environment_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_system_unittests"
       },
       {
@@ -458,6 +461,9 @@
         "test": "mojo_public_environment_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_system_unittests"
       },
       {
@@ -721,6 +727,9 @@
         "test": "mojo_public_environment_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_system_unittests"
       },
       {
@@ -984,6 +993,9 @@
         "test": "mojo_public_environment_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_system_unittests"
       },
       {
@@ -1248,6 +1260,9 @@
         "test": "mojo_public_environment_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_system_unittests"
       },
       {