[Sync] Make syncer commands avoid posting tasks on threads with no work to do

chrome/browser/sync/engine/apply_updates_command.h

 // Copyright (c) 2011 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.
 
 #ifndef CHROME_BROWSER_SYNC_ENGINE_APPLY_UPDATES_COMMAND_H_
 #define CHROME_BROWSER_SYNC_ENGINE_APPLY_UPDATES_COMMAND_H_
 #pragma once
 
 #include "base/compiler_specific.h"
 #include "chrome/browser/sync/engine/model_changing_syncer_command.h"
 
 namespace browser_sync {
 
 class ApplyUpdatesCommand : public ModelChangingSyncerCommand {
  public:
   ApplyUpdatesCommand();
   virtual ~ApplyUpdatesCommand();
 
+ protected:
   // ModelChangingSyncerCommand implementation.
+  virtual bool HasCustomGroupsToChange() const OVERRIDE;
+  virtual std::set<ModelSafeGroup> GetGroupsToChange(
+      const sessions::SyncSession& session) const OVERRIDE;
   virtual void ModelChangingExecuteImpl(
       sessions::SyncSession* session) OVERRIDE;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ApplyUpdatesCommand);
 };
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_ENGINE_APPLY_UPDATES_COMMAND_H_