Convert mojo_public_utility_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 f8c14d0bb01c09d0d094194e130ffe24d256a798..c348d2df10984e8f89c038e9736ac84c2d9400d7 100644
--- a/testing/buildbot/chromium.fyi.json
+++ b/testing/buildbot/chromium.fyi.json
@@ -464,6 +464,9 @@
         "test": "mojo_public_system_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_utility_unittests"
       },
       {
@@ -784,6 +787,9 @@
         "test": "mojo_public_system_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_utility_unittests"
       },
       {
@@ -968,6 +974,9 @@
         "test": "mojo_public_system_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_utility_unittests"
       },
       {
@@ -3373,6 +3382,9 @@
         "test": "mojo_public_system_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_utility_unittests"
       },
       {
@@ -3849,6 +3861,9 @@
         "test": "mojo_public_system_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_utility_unittests"
       },
       {