Do not observe MessageLoop destruction from ModelSafeWorker.

components/sync/driver/glue/browser_thread_model_worker_unittest.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/sync/driver/glue/browser_thread_model_worker.h"
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/location.h"
 #include "base/memory/weak_ptr.h"
@@ skipping 50 matching lines @@
   // DoWork is called first.
   void Timeout() {
     ADD_FAILURE() << "Timed out waiting for work to be done on the DB thread.";
     main_message_loop_.task_runner()->PostTask(
         FROM_HERE, base::MessageLoop::QuitWhenIdleClosure());
   }
 
  protected:
   void SetUp() override {
     db_thread_.Start();
-    worker_ = new BrowserThreadModelWorker(db_thread_.task_runner(), GROUP_DB,
-                                           nullptr);
+    worker_ = new BrowserThreadModelWorker(db_thread_.task_runner(), GROUP_DB);
   }
 
   virtual void Teardown() {
     worker_ = nullptr;
     db_thread_.Stop();
   }
 
  private:
   base::MessageLoop main_message_loop_;
   Thread db_thread_;
   bool did_do_work_;
   scoped_refptr<BrowserThreadModelWorker> worker_;
   base::OneShotTimer timer_;
 
   base::WeakPtrFactory<SyncBrowserThreadModelWorkerTest> weak_factory_;
 };
 
 TEST_F(SyncBrowserThreadModelWorkerTest, DoesWorkOnDatabaseThread) {
   base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE, base::Bind(&SyncBrowserThreadModelWorkerTest::ScheduleWork,
                             factory()->GetWeakPtr()));
   base::RunLoop().Run();
   EXPECT_TRUE(did_do_work());
 }
 
 }  // namespace
 
 }  // namespace syncer