[Sync] Adding a passthrough MetadataChangeList implementation.

components/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/buildflag_header.gni")
 import("//build/config/features.gni")
 import("//components/sync/protocol/protocol_sources.gni")
 import("//testing/test.gni")
 
 static_library("sync") {
@@ skipping 435 matching lines @@
     "model/syncable_service.cc",
     "model/syncable_service.h",
     "model/time.h",
     "model_impl/attachments/attachment_service_impl.cc",
     "model_impl/attachments/attachment_service_impl.h",
     "model_impl/attachments/task_queue.h",
     "model_impl/model_type_store_backend.cc",
     "model_impl/model_type_store_backend.h",
     "model_impl/model_type_store_impl.cc",
     "model_impl/model_type_store_impl.h",
+    "model_impl/passthrough_metadata_change_list.cc",
+    "model_impl/passthrough_metadata_change_list.h",
     "model_impl/processor_entity_tracker.cc",
     "model_impl/processor_entity_tracker.h",
     "model_impl/shared_model_type_processor.cc",
     "model_impl/shared_model_type_processor.h",
     "protocol/proto_enum_conversions.cc",
     "protocol/proto_enum_conversions.h",
     "protocol/proto_value_conversions.cc",
     "protocol/proto_value_conversions.h",
     "protocol/proto_visitors.h",
     "protocol/sync_protocol_error.cc",
@@ skipping 435 matching lines @@
     "model/mutable_data_batch_unittest.cc",
     "model/simple_metadata_change_list_unittest.cc",
     "model/sync_change_unittest.cc",
     "model/sync_data_unittest.cc",
     "model/sync_error_unittest.cc",
     "model/sync_merge_result_unittest.cc",
     "model_impl/attachments/attachment_service_impl_unittest.cc",
     "model_impl/attachments/task_queue_unittest.cc",
     "model_impl/model_type_store_backend_unittest.cc",
     "model_impl/model_type_store_impl_unittest.cc",
+    "model_impl/passthrough_metadata_change_list_unittest.cc",
     "model_impl/processor_entity_tracker_unittest.cc",
     "model_impl/shared_model_type_processor_unittest.cc",
     "protocol/proto_enum_conversions_unittest.cc",
     "protocol/proto_value_conversions_unittest.cc",
     "syncable/change_record_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",
@@ skipping 207 matching lines @@
     deps = [
       ":fake_server_jni",
       ":sync",
       ":test_support_fake_server",
       "//base",
       "//testing/gtest",
       "//url:url",
     ]
   }
 }