[sync] Add store version to model type store backend

components/sync/protocol/model_type_store_schema_descriptor.proto

Index: components/sync/protocol/model_type_store_schema_descriptor.proto
diff --git a/components/sync/protocol/model_type_store_schema_descriptor.proto b/components/sync/protocol/model_type_store_schema_descriptor.proto
new file mode 100644
index 0000000000000000000000000000000000000000..49c66ded8a1caa99c326a565dd3dd4336326da47
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+++ b/components/sync/protocol/model_type_store_schema_descriptor.proto
@@ -0,0 +1,19 @@
+// 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.
+//
+// Protocol messages used to record the state of the model type store for USS.
+// At the time of writing, the model type store uses leveldb, a schemaless
+// key-value store. This means that the database's schema is mostly implicit.
+// This descriptor isn't intended to fully describe the schema, just keep track
+// of which major changes have been applied.
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+
+package sync_pb;
+
+message ModelTypeStoreSchemaDescriptor {
+  optional int64 version_number = 1;
+}