[Sync] Allow sync start without sign-in if the local sync backend is on.

components/sync/engine_impl/sync_manager_impl.cc

Index: components/sync/engine_impl/sync_manager_impl.cc
diff --git a/components/sync/engine_impl/sync_manager_impl.cc b/components/sync/engine_impl/sync_manager_impl.cc
index 7a3466d019f9677252cc1452e80be7f5d712afa6..9d1dff5a2fd6fe96e9dcbec1c62a85f4c9c5d739 100644
--- a/components/sync/engine_impl/sync_manager_impl.cc
+++ b/components/sync/engine_impl/sync_manager_impl.cc
@@ -223,9 +223,11 @@ void SyncManagerImpl::Init(InitArgs* args) {
   CHECK(!initialized_);
   DCHECK(thread_checker_.CalledOnValidThread());
   DCHECK(args->post_factory.get());
-  DCHECK(!args->credentials.account_id.empty());
-  DCHECK(!args->credentials.sync_token.empty());
-  DCHECK(!args->credentials.scope_set.empty());
+  if (!args->enable_local_sync_backend) {
+    DCHECK(!args->credentials.account_id.empty());
+    DCHECK(!args->credentials.sync_token.empty());
+    DCHECK(!args->credentials.scope_set.empty());
+  }
   DCHECK(args->cancelation_signal);
   DVLOG(1) << "SyncManager starting Init...";
 
@@ -327,17 +329,22 @@ void SyncManagerImpl::Init(InitArgs* args) {
       listeners, &debug_info_event_listener_, model_type_registry_.get(),
       args->invalidator_client_id);
   scheduler_ = args->engine_components_factory->BuildScheduler(
-      name_, cycle_context_.get(), args->cancelation_signal);
+      name_, cycle_context_.get(), args->cancelation_signal,
+      args->enable_local_sync_backend);
 
   scheduler_->Start(SyncScheduler::CONFIGURATION_MODE, base::Time());
 
   initialized_ = true;
 
-  net::NetworkChangeNotifier::AddIPAddressObserver(this);
-  net::NetworkChangeNotifier::AddConnectionTypeObserver(this);
-  observing_network_connectivity_changes_ = true;
+  if (!args->enable_local_sync_backend) {
+    net::NetworkChangeNotifier::AddIPAddressObserver(this);
+    net::NetworkChangeNotifier::AddConnectionTypeObserver(this);
+    observing_network_connectivity_changes_ = true;
 
-  UpdateCredentials(args->credentials);
+    UpdateCredentials(args->credentials);
+  } else {
+    scheduler_->OnCredentialsUpdated();
+  }
 
   NotifyInitializationSuccess();
 }