Convert gn_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 44947776b94bc66931fee7e324e466e139898ab1..db0c0e1c1be1e6c4629948d44f05f1c37dccd453 100644
--- a/testing/buildbot/chromium.linux.json
+++ b/testing/buildbot/chromium.linux.json
@@ -559,6 +559,9 @@
         "test": "gl_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gn_unittests"
       },
       {
@@ -821,6 +824,9 @@
         "test": "gl_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gn_unittests"
       },
       {
@@ -1075,6 +1081,9 @@
         "test": "gl_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gn_unittests"
       },
       {