[login_manager] Fix thread-safety issues at shutdown

session_manager_service.cc

Index: session_manager_service.cc
diff --git a/session_manager_service.cc b/session_manager_service.cc
index d708ef1be3ff5b508b64271db640157f145f61fa..29313f30d354e70b741953a1a721c15d541722c2 100644
--- a/session_manager_service.cc
+++ b/session_manager_service.cc
@@ -328,7 +328,12 @@ bool SessionManagerService::Shutdown() {
   }
 
   // Even if we haven't gotten around to processing a persist task.
-  store_->Persist();
+  base::WaitableEvent event(true, false);
+  io_thread_.message_loop()->PostTask(
+      FROM_HERE, NewRunnableMethod(this,
+                                   &SessionManagerService::PersistStoreSync,
+                                   &event));
+  event.Wait();
   io_thread_.Stop();
   message_loop_->PostTask(FROM_HERE, new MessageLoop::QuitTask());
   LOG(INFO) << "SessionManagerService quitting run loop";
@@ -909,6 +914,12 @@ void SessionManagerService::PersistWhitelist() {
                                    what_happened));
 }
 
+void SessionManagerService::PersistStoreSync(base::WaitableEvent* event) {
+  store_->Persist();
+  LOG(INFO) << "Persisted Store to disk.";
+  event->Signal();
+}
+
 void SessionManagerService::PersistStore() {
   LOG(INFO) << "Persisting Store to disk.";
   bool what_happened = store_->Persist();