sync: Set explicit protocol_version on commit

sync/test/engine/mock_connection_manager.cc

Index: sync/test/engine/mock_connection_manager.cc
diff --git a/sync/test/engine/mock_connection_manager.cc b/sync/test/engine/mock_connection_manager.cc
index ab29b9abe246579610bc42e3cd1f164b11700984..95c00f6e6a8fcbba19a95b7a82d66bfe32e2c37b 100644
--- a/sync/test/engine/mock_connection_manager.cc
+++ b/sync/test/engine/mock_connection_manager.cc
@@ -88,6 +88,7 @@ bool MockConnectionManager::PostBufferToPath(PostBufferParams* params,
     csync::ScopedServerStatusWatcher* watcher) {
   ClientToServerMessage post;
   CHECK(post.ParseFromString(params->buffer_in));
+  CHECK(post.has_protocol_version());
   last_request_.CopyFrom(post);
   client_stuck_ = post.sync_problem_detected();
   ClientToServerResponse response;