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

chrome/browser/sync/engine/process_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_PROCESS_UPDATES_COMMAND_H_
 #define CHROME_BROWSER_SYNC_ENGINE_PROCESS_UPDATES_COMMAND_H_
 #pragma once
 
 #include "base/compiler_specific.h"
 #include "chrome/browser/sync/engine/model_changing_syncer_command.h"
@@ skipping 16 matching lines @@
 //                 and verified.
 //
 // Postconditions - All of the verified SyncEntity data will be copied to
 //                  the server fields of the corresponding syncable entries.
 // TODO(tim): This should not be ModelChanging (bug 36592).
 class ProcessUpdatesCommand : public ModelChangingSyncerCommand {
  public:
   ProcessUpdatesCommand();
   virtual ~ProcessUpdatesCommand();
 
+ protected:
   // ModelChangingSyncerCommand implementation.
-  virtual bool ModelNeutralExecuteImpl(sessions::SyncSession* session) OVERRIDE;
+  virtual std::set<ModelSafeGroup> GetGroupsToChange(
+      const sessions::SyncSession& session) const OVERRIDE;
+  virtual bool ModelNeutralExecuteImpl(
+      sessions::SyncSession* session) OVERRIDE;
   virtual void ModelChangingExecuteImpl(
       sessions::SyncSession* session) OVERRIDE;
 
  private:
   ServerUpdateProcessingResult ProcessUpdate(
       const syncable::ScopedDirLookup& dir,
       const sync_pb::SyncEntity& proto_update,
       syncable::WriteTransaction* const trans);
   DISALLOW_COPY_AND_ASSIGN(ProcessUpdatesCommand);
 };
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_ENGINE_PROCESS_UPDATES_COMMAND_H_