Sync MDP: implement MemoryDumpProvider

components/sync/protocol/proto_size_estimations.h

+// 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.
+
+#ifndef COMPONENTS_SYNC_PROTOCOL_PROTO_SIZE_ESTIMATIONS_H_
+#define COMPONENTS_SYNC_PROTOCOL_PROTO_SIZE_ESTIMATIONS_H_
+
+#include <stddef.h>
+
+namespace sync_pb {
+
+// EstimateMemoryUsage() need to be in sync_pb to be discoverable by ADL,
+// i.e. so you could just write EstimateMemoryUsage(proto) without having
+// to import EstimateMemoryUsage with 'using'. And it needs to be a template
+// to avoid listing all protos here.
+// Note: if you get linking errors you need to add explicit instantiation at
+// the end of the implementation file.
+template <class P>
+size_t EstimateMemoryUsage(const P& proto);
+
+}
+
+#endif  // COMPONENTS_SYNC_PROTOCOL_PROTO_SIZE_ESTIMATIONS_H_