Convert mojo_public_environment_unittests to run exclusively on Swarming

testing/buildbot/chromium.linux.json

Index: testing/buildbot/chromium.linux.json
diff --git a/testing/buildbot/chromium.linux.json b/testing/buildbot/chromium.linux.json
index 4004bcd37fbac14f32e9c1f432fdff1781d58e06..ab0976e26cc4663bcf1fe826cc307822a2f0fb3a 100644
--- a/testing/buildbot/chromium.linux.json
+++ b/testing/buildbot/chromium.linux.json
@@ -31,6 +31,9 @@
         "test": "mojo_public_bindings_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_environment_unittests"
       },
       {
@@ -695,6 +698,9 @@
         "test": "mojo_public_bindings_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_environment_unittests"
       },
       {
@@ -1031,6 +1037,9 @@
         "test": "mojo_public_bindings_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_environment_unittests"
       },
       {
@@ -1357,6 +1366,9 @@
         "test": "mojo_public_bindings_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_environment_unittests"
       },
       {