Brett just published a guide which suggests to avoid std::unordered_map (or at least think carefully before using it). (Please see //base/containers/README.md.)

"""But it has high memory overhead for small sizes which most of our maps are. It's also slightly slower than std::map to insert into, has poor cache coherency (every insert is a malloc), and isn't as much faster at querying over the alternatives than you might expect (it can even be slower!)."""

I wonder whether we should do some evaluation and consider changing the generated type of mojom map from std::unordered_map to something else.

The biggest reason to choose std::unordered_map was because it is more consistent with the blink-side type, which is WTF::HashMap.