[Sync] Implement MemoryDumpProvider.

components/sync/protocol/proto_memory_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_MEMORY_ESTIMATIONS_H_
+#define COMPONENTS_SYNC_PROTOCOL_PROTO_MEMORY_ESTIMATIONS_H_
+
+#include <stddef.h>
+
+namespace sync_pb {
+
+// Estimates memory usage for a proto.
+// Needs to be in sync_pb namespace for ADL to find it (when for example
+// EstimateMemoryUsage() is called on a list of protos).
+// 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);
+
+}  // namespace sync_pb
+
+#endif  // COMPONENTS_SYNC_PROTOCOL_PROTO_MEMORY_ESTIMATIONS_H_