ProxyResolutionServiceProvider in Chrome is weird. It exports a ResolveNetworkProxy D-Bus method on org.chromium.LibCrosServiceInterface, but instead of returning the proxy information in a response, it emits a signal on a caller-supplied interface.

This was implemented a long time ago, and I think it was done this way because we didn't really have a good way of making asynchronous method calls at the time. Since this code is being refactored for mus+ash, it might be a good time to also audit the callers and update Chrome to return the proxy data directly if feasible. We might be able to immediately start returning the proxy info in a response (as well as a signal) and update callers one at a time, even.