Convert sync_unit_tests to run exclusively on Swarming

testing/buildbot/chromium.mac.json

Index: testing/buildbot/chromium.mac.json
diff --git a/testing/buildbot/chromium.mac.json b/testing/buildbot/chromium.mac.json
index d25ebe2baae94fd21479e90f6115e0198f7a0346..1a1dcaef3abf96369813dcb41425b133da0ce8dc 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -213,6 +213,9 @@
         "test": "sql_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sync_unit_tests"
       },
       {
@@ -445,6 +448,9 @@
         "test": "sync_integration_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sync_unit_tests"
       },
       {
@@ -677,6 +683,9 @@
         "test": "sync_integration_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sync_unit_tests"
       },
       {
@@ -909,6 +918,9 @@
         "test": "sync_integration_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sync_unit_tests"
       },
       {
@@ -1142,6 +1154,9 @@
         "test": "sync_integration_tests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sync_unit_tests"
       },
       {