[Sync] Move thread checking into the ModelSafeWorker interface.

components/sync/driver/glue/browser_thread_model_worker.cc

 // Copyright 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/single_thread_task_runner.h"
 #include "base/synchronization/waitable_event.h"
@@ skipping 32 matching lines @@
     return error;
   }
   work_done_or_abandoned.Wait();
   return error;
 }
 
 ModelSafeGroup BrowserThreadModelWorker::GetModelSafeGroup() {
   return group_;
 }
 
+bool BrowserThreadModelWorker::IsOnModelThread() {
+  return runner_->BelongsToCurrentThread();
+}
+
 BrowserThreadModelWorker::~BrowserThreadModelWorker() {}
 
 void BrowserThreadModelWorker::CallDoWorkAndSignalTask(
     const WorkCallback& work,
     syncer::ScopedEventSignal scoped_event_signal,
     SyncerError* error) {
   DCHECK(runner_->BelongsToCurrentThread());
   if (!IsStopped())
     *error = work.Run();
   // The event in |scoped_event_signal| is signaled at the end of this scope.
 }
 
 }  // namespace syncer