[Sync] Improve MetadataChangeList usage for types using ModelTypeStore.

components/sync/model_impl/accumulating_metadata_change_list.cc

+// 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/accumulating_metadata_change_list.h"
+
+namespace syncer {
+
+AccumulatingMetadataChangeList::AccumulatingMetadataChangeList() {}
+AccumulatingMetadataChangeList::~AccumulatingMetadataChangeList() {}
+
+void AccumulatingMetadataChangeList::TransferChanges(
+    ModelTypeStore* store,
+    ModelTypeStore::WriteBatch* write_batch) {
+  DCHECK(write_batch);
+  DCHECK(store);
+  for (const auto& pair : metadata_changes_) {
+    const std::string& storage_key = pair.first;
+    const MetadataChange& change = pair.second;
+    switch (change.type) {
+      case UPDATE:
+        store->WriteMetadata(write_batch, storage_key,
+                             change.metadata.SerializeAsString());
+        break;
+      case CLEAR:
+        store->DeleteMetadata(write_batch, storage_key);
+        break;
+    }
+  }
+  metadata_changes_.clear();
+  if (state_change_) {
+    switch (state_change_->type) {
+      case UPDATE:
+        store->WriteGlobalMetadata(write_batch,
+                                   state_change_->state.SerializeAsString());
+        break;
+      case CLEAR:
+        store->DeleteGlobalMetadata(write_batch);
+        break;
+    }
+    state_change_.reset();
+  }
+}
+
+}  // namespace syncer