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

chrome/browser/sync/engine/process_updates_command.cc

 // 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.
 
 #include "chrome/browser/sync/engine/process_updates_command.h"
 
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/location.h"
 #include "chrome/browser/sync/engine/syncer.h"
 #include "chrome/browser/sync/engine/syncer_proto_util.h"
 #include "chrome/browser/sync/engine/syncer_util.h"
 #include "chrome/browser/sync/engine/syncproto.h"
 #include "chrome/browser/sync/sessions/sync_session.h"
 #include "chrome/browser/sync/syncable/directory_manager.h"
 #include "chrome/browser/sync/syncable/syncable.h"
 
 using std::vector;
 
 namespace browser_sync {
 
 using sessions::SyncSession;
 using sessions::StatusController;
+using sessions::UpdateProgress;
 
 ProcessUpdatesCommand::ProcessUpdatesCommand() {}
 ProcessUpdatesCommand::~ProcessUpdatesCommand() {}
 
+bool ProcessUpdatesCommand::HasCustomGroupsToChange() const {
+  // TODO(akalin): Set to true.
+  return false;
+}
+
+std::set<ModelSafeGroup> ProcessUpdatesCommand::GetGroupsToChange(
+    const sessions::SyncSession& session) const {
+  return session.GetEnabledGroupsWithVerifiedUpdates();
+}
+
 bool ProcessUpdatesCommand::ModelNeutralExecuteImpl(SyncSession* session) {
   const GetUpdatesResponse& updates =
       session->status_controller().updates_response().get_updates();
   const int update_count = updates.entries_size();
 
   // Don't bother processing updates if there were none.
   return update_count != 0;
 }
 
 void ProcessUpdatesCommand::ModelChangingExecuteImpl(SyncSession* session) {
@@ skipping 107 matching lines @@
     // Force application of this update, no matter what.
     target_entry.Put(syncable::IS_UNAPPLIED_UPDATE, true);
   }
 
   SyncerUtil::UpdateServerFieldsFromUpdate(&target_entry, update, name);
 
   return SUCCESS_PROCESSED;
 }
 
 }  // namespace browser_sync