Merge pull request #3348 from matthauck/fix-gcc41-again

Fix map_field_inl.h for gcc 4.1

src/google/protobuf/map_field_inl.h

@@ -252,9 +252,9 @@ template <typename Derived, typename Key, typename T,
           WireFormatLite::FieldType kValueFieldType, int default_enum_value>
 void MapField<Derived, Key, T, kKeyFieldType, kValueFieldType,
               default_enum_value>::Swap(MapField* other) {
-  std::swap(MapFieldBase::repeated_field_, other->repeated_field_);
+  std::swap(this->MapFieldBase::repeated_field_, other->repeated_field_);
   impl_.Swap(&other->impl_);
-  std::swap(MapFieldBase::state_, other->state_);
+  std::swap(this->MapFieldBase::state_, other->state_);
 }
 
 template <typename Derived, typename Key, typename T,