sync: Add pre-commit update avoidance mode + flag

sync/sessions/sync_session_context.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/sessions/sync_session_context.h"
 
 #include "sync/sessions/debug_info_getter.h"
 #include "sync/util/extensions_activity_monitor.h"
 
 namespace syncer {
 namespace sessions {
 
 const unsigned int kMaxMessagesToRecord = 10;
 const unsigned int kMaxMessageSizeToRecord = 5 * 1024;
 
 SyncSessionContext::SyncSessionContext(
     ServerConnectionManager* connection_manager,
     syncable::Directory* directory,
     const std::vector<ModelSafeWorker*>& workers,
     ExtensionsActivityMonitor* extensions_activity_monitor,
     const std::vector<SyncEngineEventListener*>& listeners,
     DebugInfoGetter* debug_info_getter,
     TrafficRecorder* traffic_recorder,
     bool keystore_encryption_enabled,
+    bool force_enable_pre_commit_update_avoidance,
     const std::string& invalidator_client_id)
     : connection_manager_(connection_manager),
       directory_(directory),
       extensions_activity_monitor_(extensions_activity_monitor),
       notifications_enabled_(false),
       max_commit_batch_size_(kDefaultMaxCommitBatchSize),
       debug_info_getter_(debug_info_getter),
       traffic_recorder_(traffic_recorder),
       keystore_encryption_enabled_(keystore_encryption_enabled),
-      invalidator_client_id_(invalidator_client_id) {
+      invalidator_client_id_(invalidator_client_id),
+      pre_commit_update_avoidance_experiment_enabled_(false),
+      force_enable_pre_commit_update_avoidance_(
+          force_enable_pre_commit_update_avoidance) {
   for (size_t i = 0u; i < workers.size(); ++i)
     workers_.push_back(workers[i]);
 
   std::vector<SyncEngineEventListener*>::const_iterator it;
   for (it = listeners.begin(); it != listeners.end(); ++it)
     listeners_.AddObserver(*it);
 }
 
 SyncSessionContext::~SyncSessionContext() {
 }
 
 }  // namespace sessions
 }  // namespace syncer