Implement utility-side proxy resolver Mojo service.

net/interfaces/proxy_resolver_service.mojom

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Put Mojo definitions into their own namespace to avoid collisions with C++
 // definitions.
 // TODO(amistry): Resolve the conflict between these two sets of definitions.
 module net.interfaces;
 
 import "host_resolver_service.mojom";
 
 // Mirror of net::ProxyServer::Scheme.
 enum ProxyScheme {
   INVALID,
   DIRECT,
   HTTP,
   SOCKS4,
   SOCKS5,
   HTTPS,
   QUIC,
 };
 
 // Mirror of net::ProxyServer.
 struct ProxyServer {
   ProxyScheme scheme;
-  string host;
+
+  // |host| and |port| are only valid if |scheme| is not INVALID or DIRECT.
+  string? host;
   uint16 port;
 };
 
 interface ProxyResolver {
   SetPacScript(string data) => (int32 result);
 
   // Use a ProxyResolverRequestClient instead of returning a result so we can
   // receive load state updates and cancel in-flight requests by destroying the
   // client.
   // TODO(amistry): Add BoundNetLog.
   GetProxyForUrl(string url, ProxyResolverRequestClient client);
 };
 
 interface ProxyResolverRequestClient {
   ReportResult(int32 error, array<ProxyServer>? proxy_servers);
 };
 
 interface ProxyResolverFactory {
   // TODO(amistry): Add NetLog and ProxyResolverErrorObserver.
   CreateResolver(ProxyResolver& resolver,
                  HostResolver host_resolver);
 };