[Sync] Implement MemoryDumpProvider.

components/sync/base/proto_value_ptr.h

Index: components/sync/base/proto_value_ptr.h
diff --git a/components/sync/base/proto_value_ptr.h b/components/sync/base/proto_value_ptr.h
index 35c86b346aa199fd1e8eeb4695c56700f90fa05d..89189b9412d383c88fe31f68d65338c7e4b32710 100644
--- a/components/sync/base/proto_value_ptr.h
+++ b/components/sync/base/proto_value_ptr.h
@@ -125,6 +125,13 @@ class ProtoValuePtr {
   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_