Convert mojo_public_environment_unittests to run exclusively on Swarming

testing/buildbot/chromium.fyi.json

Index: testing/buildbot/chromium.fyi.json
diff --git a/testing/buildbot/chromium.fyi.json b/testing/buildbot/chromium.fyi.json
index 4ff6efb260025ff441a044b442de789772cdc5d6..7a10eaa4983dd9ffbe08b9aa75512b024af3e8fd 100644
--- a/testing/buildbot/chromium.fyi.json
+++ b/testing/buildbot/chromium.fyi.json
@@ -452,6 +452,9 @@
         "test": "mojo_public_bindings_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_environment_unittests"
       },
       {
@@ -766,6 +769,9 @@
         "test": "mojo_public_bindings_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_environment_unittests"
       },
       {
@@ -944,6 +950,9 @@
         "test": "mojo_public_bindings_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_environment_unittests"
       },
       {
@@ -3343,6 +3352,9 @@
         "test": "mojo_public_bindings_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_environment_unittests"
       },
       {
@@ -3813,6 +3825,9 @@
         "test": "mojo_public_bindings_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_environment_unittests"
       },
       {