sync: Add pre-commit update avoidance mode + flag

sync/internal_api/sync_manager_impl.cc

Index: sync/internal_api/sync_manager_impl.cc
diff --git a/sync/internal_api/sync_manager_impl.cc b/sync/internal_api/sync_manager_impl.cc
index e9eeea183217b5458ca3b7da67176eb376c24dbf..3d6158d8a15dc8506a635ad8041dbf9c031cdbb4 100644
--- a/sync/internal_api/sync_manager_impl.cc
+++ b/sync/internal_api/sync_manager_impl.cc
@@ -1295,6 +1295,17 @@ bool SyncManagerImpl::ReceivedExperiment(Experiments* experiments) {
     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;
 }