Do not observe MessageLoop destruction from ModelSafeWorker.

components/sync/test/engine/fake_model_worker.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/test/engine/fake_model_worker.h"
 
+#include "base/callback.h"
+
 namespace syncer {
 
-FakeModelWorker::FakeModelWorker(ModelSafeGroup group)
-    : ModelSafeWorker(nullptr), group_(group) {}
+FakeModelWorker::FakeModelWorker(ModelSafeGroup group) : group_(group) {}
 
 FakeModelWorker::~FakeModelWorker() {
   // We may need to relax this is FakeModelWorker is used in a
   // multi-threaded test; since ModelSafeWorkers are
   // RefCountedThreadSafe, they could theoretically be destroyed from
   // a different thread.
   DCHECK(CalledOnValidThread());
 }
 
-void FakeModelWorker::RegisterForLoopDestruction() {
-  NOTREACHED();
-}
-
 SyncerError FakeModelWorker::DoWorkAndWaitUntilDoneImpl(
     const WorkCallback& work) {
   DCHECK(CalledOnValidThread());
   // Simply do the work on the current thread.
   return work.Run();
 }
 
 ModelSafeGroup FakeModelWorker::GetModelSafeGroup() {
   DCHECK(CalledOnValidThread());
   return group_;
 }
 
 }  // namespace syncer