Update Commit and GetUpdatesResponse messages to include attachment ids.

sync/internal_api/syncapi_internal.h

Index: sync/internal_api/syncapi_internal.h
diff --git a/sync/internal_api/syncapi_internal.h b/sync/internal_api/syncapi_internal.h
index 2e7965de5ce21b5897b59532cb4b2bc12e1619f2..acb1631be4989bfc96d9f2648720270a3de876c6 100644
--- a/sync/internal_api/syncapi_internal.h
+++ b/sync/internal_api/syncapi_internal.h
@@ -13,6 +13,7 @@
 #include "sync/base/sync_export.h"
 
 namespace sync_pb {
+class AttachmentMetadata;
 class EntitySpecifics;
 class PasswordSpecificsData;
 }
@@ -35,6 +36,11 @@ bool IsNameServerIllegalAfterTrimming(const std::string& name);
 bool AreSpecificsEqual(const Cryptographer* cryptographer,
                        const sync_pb::EntitySpecifics& left,
                        const sync_pb::EntitySpecifics& right);
+
+// Return true iff |left| and |right| are equal.
+bool AreAttachmentMetadataEqual(const sync_pb::AttachmentMetadata& left,
+                                const sync_pb::AttachmentMetadata& right);
+
 }  // namespace syncer
 
 #endif  // SYNC_INTERNAL_API_SYNCAPI_INTERNAL_H_