Convert mojo_public_environment_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 400370814675d718df09ce02e7a68278a6f69545..2327747bed4c1774aa967ba8b920eb883fa92b63 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -198,6 +198,9 @@
         "test": "mojo_public_bindings_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_environment_unittests"
       },
       {
@@ -449,6 +452,9 @@
         "test": "mojo_public_bindings_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_environment_unittests"
       },
       {
@@ -709,6 +715,9 @@
         "test": "mojo_public_bindings_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_environment_unittests"
       },
       {
@@ -969,6 +978,9 @@
         "test": "mojo_public_bindings_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_environment_unittests"
       },
       {
@@ -1230,6 +1242,9 @@
         "test": "mojo_public_bindings_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_environment_unittests"
       },
       {