[Sync] Implement DummyMetadataChangelist and use it in SharingMessageBridge.

DummyMetadataChangelist may be useful for data types which do not store
any data in the bridge.

Bug: 1034930
Change-Id: I2cb2f548f1874bba1edb8fa04afe31b15f0b42d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2019206
Commit-Queue: Rushan Suleymanov <rushans@google.com>
Reviewed-by: vitaliii <vitaliii@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735421}

chrome/browser/sharing/sharing_message_bridge_impl.cc

@@ -7,7 +7,7 @@
 #include "base/guid.h"
 #include "components/sync/model/metadata_batch.h"
 #include "components/sync/model/mutable_data_batch.h"
-#include "components/sync/model_impl/in_memory_metadata_change_list.h"
+#include "components/sync/model_impl/dummy_metadata_change_list.h"
 
 namespace {
 
@@ -67,10 +67,7 @@ std::unique_ptr<syncer::MetadataChangeList>
 SharingMessageBridgeImpl::CreateMetadataChangeList() {
   // The data type intentionally doesn't persist the data on disk, so metadata
   // is just ignored.
-  // TODO(crbug.com/1034930): this metadata changelist stores data in memory, it
-  // would be better to create DummyMetadataChangeList to ignore any changes at
-  // all.
-  return std::make_unique<syncer::InMemoryMetadataChangeList>();
+  return std::make_unique<syncer::DummyMetadataChangeList>();
 }
 
 base::Optional<syncer::ModelError> SharingMessageBridgeImpl::MergeSyncData(

components/sync/BUILD.gn

@@ -290,6 +290,8 @@ jumbo_static_library("rest_of_sync") {
     "model_impl/client_tag_based_model_type_processor.h",
     "model_impl/client_tag_based_remote_update_handler.cc",
     "model_impl/client_tag_based_remote_update_handler.h",
+    "model_impl/dummy_metadata_change_list.cc",
+    "model_impl/dummy_metadata_change_list.h",
     "model_impl/forwarding_model_type_controller_delegate.cc",
     "model_impl/forwarding_model_type_controller_delegate.h",
     "model_impl/in_memory_metadata_change_list.cc",

components/sync/model_impl/dummy_metadata_change_list.cc

+// Copyright 2020 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/dummy_metadata_change_list.h"
+
+namespace syncer {
+
+DummyMetadataChangeList::DummyMetadataChangeList() = default;
+DummyMetadataChangeList::~DummyMetadataChangeList() = default;
+
+void DummyMetadataChangeList::UpdateModelTypeState(
+    const sync_pb::ModelTypeState& model_type_state) {}
+
+void DummyMetadataChangeList::ClearModelTypeState() {}
+
+void DummyMetadataChangeList::UpdateMetadata(
+    const std::string& storage_key,
+    const sync_pb::EntityMetadata& metadata) {}
+
+void DummyMetadataChangeList::ClearMetadata(const std::string& storage_key) {}
+
+}  // namespace syncer

components/sync/model_impl/dummy_metadata_change_list.h

+// Copyright 2020 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.
+
+#ifndef COMPONENTS_SYNC_MODEL_IMPL_DUMMY_METADATA_CHANGE_LIST_H_
+#define COMPONENTS_SYNC_MODEL_IMPL_DUMMY_METADATA_CHANGE_LIST_H_
+
+#include <string>
+
+#include "components/sync/model/metadata_change_list.h"
+
+namespace syncer {
+
+// A MetadataChangeList class that does not store anything.
+class DummyMetadataChangeList : public MetadataChangeList {
+ public:
+  DummyMetadataChangeList();
+  ~DummyMetadataChangeList() override;
+
+  // MetadataChangeList implementation.
+  void UpdateModelTypeState(
+      const sync_pb::ModelTypeState& model_type_state) override;
+  void ClearModelTypeState() override;
+  void UpdateMetadata(const std::string& storage_key,
+                      const sync_pb::EntityMetadata& metadata) override;
+  void ClearMetadata(const std::string& storage_key) override;
+};
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_MODEL_IMPL_DUMMY_METADATA_CHANGE_LIST_H_