[Sync] Implement MemoryDumpProvider.

components/sync/base/proto_value_ptr.h

 // Copyright 2015 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_BASE_PROTO_VALUE_PTR_H_
 #define COMPONENTS_SYNC_BASE_PROTO_VALUE_PTR_H_
 
 #include "base/gtest_prod_util.h"
 #include "base/memory/ref_counted.h"
 
@@ skipping 107 matching lines @@
     }
   }
 
   void load(const void* blob, int length) {
     wrapper_ = Wrapper::ParseFromBlob(blob, length);
   }
 
   scoped_refptr<Wrapper> wrapper_;
 };
 
+template <typename T, typename Traits>
+size_t EstimateMemoryUsage(const ProtoValuePtr<T, Traits>& ptr) {
+  return &ptr.value() != &Traits::DefaultValue()
+             ? EstimateMemoryUsage(ptr.value())
+             : 0;
+}
+
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_BASE_PROTO_VALUE_PTR_H_