[Sync] Move thread checking into the ModelSafeWorker interface.

components/sync/engine/passive_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/engine/passive_model_worker.h"
 
 #include "base/callback.h"
 
 namespace syncer {
 
 PassiveModelWorker::PassiveModelWorker() = default;
 
 PassiveModelWorker::~PassiveModelWorker() {}
 
 SyncerError PassiveModelWorker::DoWorkAndWaitUntilDoneImpl(
     const WorkCallback& work) {
   // Simply do the work on the current thread.
   return work.Run();
 }
 
 ModelSafeGroup PassiveModelWorker::GetModelSafeGroup() {
   return GROUP_PASSIVE;
 }
 
+bool PassiveModelWorker::IsOnModelThread() {
+  NOTREACHED();

[Nicolas Zea, 2016/11/08 18:31:04]

nit: maybe add a comment here about why this is invalid to call?

[maxbogue, 2016/11/08 18:43:23]

Done.

+  return false;
+}
+
 }  // namespace syncer