Add per attachment metadata records.

sync/internal_api/attachments/proto/attachment_store.proto

Index: sync/internal_api/attachments/proto/attachment_store.proto
diff --git a/sync/internal_api/attachments/proto/attachment_store.proto b/sync/internal_api/attachments/proto/attachment_store.proto
index 533332d5d011417ff241c0e86d97803319c4fb30..86f90715593464118bbf87c27637731512116fc2 100644
--- a/sync/internal_api/attachments/proto/attachment_store.proto
+++ b/sync/internal_api/attachments/proto/attachment_store.proto
@@ -10,9 +10,18 @@ option retain_unknown_fields = true;
 package attachment_store_pb;
 
 // Metadata for leveldb attachment store database.
-message AttachmentStoreMetadata {
+message StoreMetadata {
   // |schema_version| indicates format in which data is written in attachment
   // store. Needed for upgrade and to prevent newer data from being loaded by
   // older code that doesn't understand it.
   optional int32 schema_version = 1;
 }
+
+// Metadata for attachment in attachment store. Storing metadata in separate
+// record from actual data allows us to enumerate attachments in the store
+// without incurring cost to read actual data. It also allows us to update
+// attachment metadata independent of the data.
+message RecordMetadata {
+  // Size of attachment data. Useful for attachment store space management.
+  optional int64 attachment_size = 1;
+}