Convert sbox_unittests to run exclusively on Swarming

testing/buildbot/chromium.win.json

Index: testing/buildbot/chromium.win.json
diff --git a/testing/buildbot/chromium.win.json b/testing/buildbot/chromium.win.json
index 85d5dfdd1e4f8a55f6001767a4cd4596ddec4847..0ae1d2c36e7d75ec3bbc9cec9058c44c8ff1692a 100644
--- a/testing/buildbot/chromium.win.json
+++ b/testing/buildbot/chromium.win.json
@@ -223,6 +223,9 @@
         "test": "sbox_integration_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sbox_unittests"
       },
       {
@@ -545,6 +548,9 @@
         "test": "sbox_integration_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sbox_unittests"
       },
       {
@@ -986,6 +992,9 @@
         "test": "sbox_integration_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sbox_unittests"
       },
       {
@@ -1281,6 +1290,9 @@
         "test": "sbox_integration_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sbox_unittests"
       },
       {
@@ -1738,6 +1750,9 @@
         "test": "sbox_integration_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sbox_unittests"
       },
       {