The URLLoaderFactoryBundle mojo struct has:
  // The default factory to be used when no others apply.
  network.mojom.URLLoaderFactory default_factory;

  // A mapping from URL scheme to factory interface.
  map<string, network.mojom.URLLoaderFactory> factories;

We can simplify it by having only a map. The reason a default_factory exists is because then we can send one mojo pipe for all the schemes handled by the network process, when default_factory points to it. In previous brainstormings when this was added, Ken had suggested having a map keyed by array<string>. That way we'd still send just one pipe to the network process. This would simplify the renderer side a little, and add consistency as well as to how it looks up factories.