sync: Add pre-commit update avoidance mode + flag

sync/internal_api/sync_manager_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/sync_manager_impl.h"
 
 #include <string>
 
 #include "base/base64.h"
 #include "base/bind.h"
@@ skipping 1277 matching lines @@
   ReadNode favicon_sync_node(&trans);
   if (favicon_sync_node.InitByClientTagLookup(
           syncer::EXPERIMENTS,
           syncer::kFaviconSyncTag) == BaseNode::INIT_OK) {
     experiments->favicon_sync_limit =
         favicon_sync_node.GetExperimentsSpecifics().favicon_sync().
             favicon_sync_limit();
     found_experiment = true;
   }
 
+  ReadNode pre_commit_update_avoidance_node(&trans);
+  if (pre_commit_update_avoidance_node.InitByClientTagLookup(
+          syncer::EXPERIMENTS,
+          syncer::kPreCommitUpdateAvoidanceTag) == BaseNode::INIT_OK) {
+    session_context_->set_server_enabled_pre_commit_update_avoidance(
+        pre_commit_update_avoidance_node.GetExperimentsSpecifics().
+            pre_commit_update_avoidance().enabled());
+    // We don't bother setting found_experiment.  The frontend doesn't need to
+    // know about this.
+  }
+
   return found_experiment;
 }
 
 bool SyncManagerImpl::HasUnsyncedItems() {
   ReadTransaction trans(FROM_HERE, GetUserShare());
   return (trans.GetWrappedTrans()->directory()->unsynced_entity_count() != 0);
 }
 
 SyncEncryptionHandler* SyncManagerImpl::GetEncryptionHandler() {
   return sync_encryption_handler_.get();
 }
 
 // static.
 int SyncManagerImpl::GetDefaultNudgeDelay() {
   return kDefaultNudgeDelayMilliseconds;
 }
 
 // static.
 int SyncManagerImpl::GetPreferencesNudgeDelay() {
   return kPreferencesNudgeDelayMilliseconds;
 }
 
 }  // namespace syncer