Do not observe MessageLoop destruction from ModelSafeWorker.

components/sync/driver/glue/ui_model_worker_unittest.cc

Index: components/sync/driver/glue/ui_model_worker_unittest.cc
diff --git a/components/sync/driver/glue/ui_model_worker_unittest.cc b/components/sync/driver/glue/ui_model_worker_unittest.cc
index 2c83554c25b8938b46499edbfca9a60610d8a873..d091f317a5c60f2896c0a05ec84abbea9095a730 100644
--- a/components/sync/driver/glue/ui_model_worker_unittest.cc
+++ b/components/sync/driver/glue/ui_model_worker_unittest.cc
@@ -36,7 +36,7 @@ class SyncUIModelWorkerTest : public testing::Test {
  public:
   SyncUIModelWorkerTest() : sync_thread_("SyncThreadForTest") {
     sync_thread_.Start();
-    worker_ = new UIModelWorker(base::ThreadTaskRunnerHandle::Get(), nullptr);
+    worker_ = new UIModelWorker(base::ThreadTaskRunnerHandle::Get());
   }
 
   void PostWorkToSyncThread(WorkCallback work) {