[Sync] Adding storage key concept for ModelTypeServices.

components/sync/api/metadata_batch.cc

Index: components/sync/api/metadata_batch.cc
diff --git a/components/sync/api/metadata_batch.cc b/components/sync/api/metadata_batch.cc
index f55ab8ef0bc19c852668852eac2551ddf51f897a..6402da40a44fb0ab3a4e5736a229f966f2c6afce 100644
--- a/components/sync/api/metadata_batch.cc
+++ b/components/sync/api/metadata_batch.cc
@@ -4,6 +4,8 @@
 
 #include "components/sync/api/metadata_batch.h"
 
+#include <utility>
+
 namespace syncer_v2 {
 
 MetadataBatch::MetadataBatch() {}
@@ -13,9 +15,9 @@ EntityMetadataMap&& MetadataBatch::TakeAllMetadata() {
   return std::move(metadata_map_);
 }
 
-void MetadataBatch::AddMetadata(const std::string& client_tag,
+void MetadataBatch::AddMetadata(const std::string& storage_key,
                                 const sync_pb::EntityMetadata& metadata) {
-  metadata_map_.insert(std::make_pair(client_tag, metadata));
+  metadata_map_.insert(std::make_pair(storage_key, metadata));
 }
 
 const sync_pb::DataTypeState& MetadataBatch::GetDataTypeState() const {