sync: Add pre-commit update avoidance mode + flag

sync/internal_api/internal_components_factory_impl.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 "sync/internal_api/public/internal_components_factory_impl.h"
 
 #include "sync/engine/backoff_delay_provider.h"
 #include "sync/engine/syncer.h"
 #include "sync/engine/sync_scheduler_impl.h"
 #include "sync/sessions/sync_session_context.h"
@@ skipping 30 matching lines @@
     const std::vector<SyncEngineEventListener*>& listeners,
     sessions::DebugInfoGetter* debug_info_getter,
     TrafficRecorder* traffic_recorder,
     const std::string& invalidation_client_id) {
   return scoped_ptr<sessions::SyncSessionContext>(
       new sessions::SyncSessionContext(
           connection_manager, directory, workers, monitor,
           listeners, debug_info_getter,
           traffic_recorder,
           switches_.encryption_method == ENCRYPTION_KEYSTORE,
+          switches_.pre_commit_updates_policy_override ==
+              FORCE_ENABLE_PRE_COMMIT_UPDATE_AVOIDANCE,
           invalidation_client_id));
 }
 
 scoped_ptr<syncable::DirectoryBackingStore>
 InternalComponentsFactoryImpl::BuildDirectoryBackingStore(
       const std::string& dir_name, const base::FilePath& backing_filepath) {
   return scoped_ptr<syncable::DirectoryBackingStore>(
       new syncable::OnDiskDirectoryBackingStore(dir_name, backing_filepath));
 }
 
 InternalComponentsFactory::Switches
 InternalComponentsFactoryImpl::GetSwitches() const {
   return switches_;
 }
 
 }  // namespace syncer