Fix SyncManager initialization failure crash.

sync/syncable/invalid_directory_backing_store.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/syncable/invalid_directory_backing_store.h"
+
+namespace syncer {
+namespace syncable {
+
+InvalidDirectoryBackingStore::InvalidDirectoryBackingStore()
+    : DirectoryBackingStore("some_fake_user") {
+}
+
+DirOpenResult InvalidDirectoryBackingStore::Load(
+    MetahandlesIndex* entry_bucket,
+    Directory::KernelLoadInfo* kernel_load_info) {
+  return FAILED_OPEN_DATABASE;
+}
+
+}  // namespace syncable
+}  // namespace syncer