Let SyncBackupManager keep backup data in memory until shutdown. Only persist

sync/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/features.gni")
 
 component("sync") {
   deps = [
     ":sync_core",
     "//sync/protocol",
@@ skipping 293 matching lines @@
     "sessions/model_type_registry.h",
     "sessions/nudge_tracker.cc",
     "sessions/nudge_tracker.h",
     "sessions/status_controller.cc",
     "sessions/status_controller.h",
     "sessions/sync_session.cc",
     "sessions/sync_session_context.cc",
     "sessions/sync_session_context.h",
     "sessions/sync_session.h",
     "syncable/blob.h",
+    "syncable/deferred_on_disk_directory_backing_store.cc",
+    "syncable/deferred_on_disk_directory_backing_store.h",
     "syncable/directory_backing_store.cc",
     "syncable/directory_backing_store.h",
     "syncable/directory.cc",
     "syncable/directory_change_delegate.h",
     "syncable/directory.h",
     "syncable/dir_open_result.h",
     "syncable/entry.cc",
     "syncable/entry.h",
     "syncable/entry_kernel.cc",
     "syncable/entry_kernel.h",
@@ skipping 284 matching lines @@
     "internal_api/sync_rollback_manager_base_unittest.cc",
     "internal_api/sync_rollback_manager_unittest.cc",
     "internal_api/syncapi_server_connection_manager_unittest.cc",
     "js/js_event_details_unittest.cc",
     "js/sync_js_controller_unittest.cc",
     "protocol/proto_enum_conversions_unittest.cc",
     "protocol/proto_value_conversions_unittest.cc",
     "sessions/model_type_registry_unittest.cc",
     "sessions/nudge_tracker_unittest.cc",
     "sessions/status_controller_unittest.cc",
+    "syncable/deferred_on_disk_directory_backing_store_unittest.cc",
     "syncable/directory_backing_store_unittest.cc",
     "syncable/directory_unittest.cc",
     "syncable/directory_unittest.h",
     "syncable/entry_kernel_unittest.cc",
     "syncable/model_type_unittest.cc",
     "syncable/nigori_util_unittest.cc",
     "syncable/parent_child_index_unittest.cc",
     "syncable/syncable_enum_conversions_unittest.cc",
     "syncable/syncable_id_unittest.cc",
     "syncable/syncable_unittest.cc",
@@ skipping 141 matching lines @@
       "//base",
       "//base/test:test_support",
       "//net:test_support",
       "//testing/gtest",
       ":test_support_sync_testserver",
     ]
   }
 }
 
 #TODO(GYP): Need to port sync_android.gypi and Android test targets.