Convert mojo_public_system_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 7a10eaa4983dd9ffbe08b9aa75512b024af3e8fd..f8c14d0bb01c09d0d094194e130ffe24d256a798 100644
--- a/testing/buildbot/chromium.fyi.json
+++ b/testing/buildbot/chromium.fyi.json
@@ -458,6 +458,9 @@
         "test": "mojo_public_environment_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_system_unittests"
       },
       {
@@ -775,6 +778,9 @@
         "test": "mojo_public_environment_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_system_unittests"
       },
       {
@@ -956,6 +962,9 @@
         "test": "mojo_public_environment_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_system_unittests"
       },
       {
@@ -3358,6 +3367,9 @@
         "test": "mojo_public_environment_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_system_unittests"
       },
       {
@@ -3831,6 +3843,9 @@
         "test": "mojo_public_environment_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "mojo_public_system_unittests"
       },
       {