Move mojom definitions of URL{Request,Response} into SDK

sky/tests/services/network.sky

 #!mojo mojo:sky_viewer
 <sky>
 <script>
 import '/sky/framework/embedder.dart';
 import 'dart:async';
 import 'dart:typed_data';
 import "dart:sky.internals" as internals;
 
 import 'package:mojo/public/dart/bindings.dart';
 import 'package:mojo/public/dart/core.dart';
 import 'package:mojom/mojo/network_service.mojom.dart';
+import 'package:mojom/mojo/url_request.mojom.dart';
 import 'package:mojom/mojo/url_loader.mojom.dart';
 
 Future<String> run(url) async {
   var networkService= new NetworkServiceProxy.unbound();
   embedder.connectToService("mojo:network_service", networkService);
 
   var urlLoader = new UrlLoaderProxy.unbound();
   networkService.ptr.createUrlLoader(urlLoader);
 
   var urlRequest = new UrlRequest()
@@ skipping 15 matching lines @@
   return new String.fromCharCodes(new Uint8List.view(bodyData.buffer));
 }
 
 main() async {
   var url = "http://127.0.0.1:8000/sky/tests/services/resources/pass.txt";
   var result = await run(url);
   internals.notifyTestComplete(result);
 }
 </script>
 </sky>