Sync MDP: 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 0f97323ea9927bebe93153a452cc8392ae89d0e6..a10f19f21aa3cc1921ffe45f9c59a83dfe0a8cc3 100644
--- a/components/sync/base/proto_value_ptr.h
+++ b/components/sync/base/proto_value_ptr.h
@@ -89,6 +89,10 @@ class ProtoValuePtr {
     return &wrapped_instance;
   }
 
+  explicit operator bool () const { return wrapper_ != nullptr; }
+  bool operator == (decltype(nullptr)) const { return wrapper_ == nullptr; }
+  bool operator != (decltype(nullptr)) const { return wrapper_ != nullptr; }
+
  private:
   friend struct syncable::EntryKernel;
   friend struct syncer_v2::EntityData;
@@ -127,6 +131,11 @@ class ProtoValuePtr {
   scoped_refptr<Wrapper> wrapper_;
 };
 
+template <typename T, typename Traits>
+size_t EstimateMemoryUsage(const ProtoValuePtr<T, Traits>& ptr) {
+  return ptr? EstimateMemoryUsage(ptr.value()) : 0;
+}
+
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_BASE_PROTO_VALUE_PTR_H_