[Sync] Adding a passthrough MetadataChangeList implementation.

components/sync/BUILD.gn

Index: components/sync/BUILD.gn
diff --git a/components/sync/BUILD.gn b/components/sync/BUILD.gn
index bec1f8ddb7a92351f8f4cb79922e9cfc5423c520..671726d3076c63ae2964dcd4f6545c7257ee17d5 100644
--- a/components/sync/BUILD.gn
+++ b/components/sync/BUILD.gn
@@ -453,6 +453,8 @@ static_library("sync") {
     "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",
@@ -908,6 +910,7 @@ source_set("unit_tests") {
     "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",