[Sync] Improve MetadataChangeList usage for types using ModelTypeStore.

components/sync/model_impl/in_memory_metadata_change_list.cc

Index: components/sync/model_impl/in_memory_metadata_change_list.cc
diff --git a/components/sync/model_impl/in_memory_metadata_change_list.cc b/components/sync/model_impl/in_memory_metadata_change_list.cc
new file mode 100644
index 0000000000000000000000000000000000000000..23c2a947fc11145a210134cbfbf91421fbd8fefe
--- /dev/null
+++ b/components/sync/model_impl/in_memory_metadata_change_list.cc
@@ -0,0 +1,31 @@
+// Copyright 2016 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 "components/sync/model_impl/in_memory_metadata_change_list.h"
+
+namespace syncer {
+
+InMemoryMetadataChangeList::InMemoryMetadataChangeList() {}
+InMemoryMetadataChangeList::~InMemoryMetadataChangeList() {}
+
+void InMemoryMetadataChangeList::UpdateModelTypeState(
+    const sync_pb::ModelTypeState& model_type_state) {
+  state_change_.reset(new ModelTypeStateChange{UPDATE, model_type_state});
+}
+
+void InMemoryMetadataChangeList::ClearModelTypeState() {
+  state_change_.reset(new ModelTypeStateChange{CLEAR});
+}
+
+void InMemoryMetadataChangeList::UpdateMetadata(
+    const std::string& storage_key,
+    const sync_pb::EntityMetadata& metadata) {
+  metadata_changes_[storage_key] = {UPDATE, metadata};
+}
+
+void InMemoryMetadataChangeList::ClearMetadata(const std::string& storage_key) {
+  metadata_changes_[storage_key] = {CLEAR, sync_pb::EntityMetadata()};
+}
+
+}  // namespace syncer