Convert sync_unit_tests 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 fd721410ad261c50f0bf2bd96a1b92052c5569f3..c3d004ebbd5eff64aae41f6a18c6d3a35269c0a3 100644
--- a/testing/buildbot/chromium.win.json
+++ b/testing/buildbot/chromium.win.json
@@ -226,6 +226,9 @@
         "test": "sync_integration_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sync_unit_tests"
       },
       {
@@ -518,6 +521,9 @@
         "test": "sync_integration_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sync_unit_tests"
       },
       {
@@ -929,6 +935,9 @@
         "test": "sync_integration_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sync_unit_tests"
       },
       {
@@ -1201,6 +1210,9 @@
         "test": "sync_integration_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sync_unit_tests"
       },
       {
@@ -1630,6 +1642,9 @@
         "test": "sync_integration_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sync_unit_tests"
       },
       {