Introduce Http{Request,Response} mojom structs.

mojo/services/network/public/interfaces/url_loader.mojom

 // Copyright 2014 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.
 
 module mojo;
 
+import "network/public/interfaces/http_message.mojom";
 import "network/public/interfaces/network_error.mojom";
 
-struct HTTPHeader {
-  string name;
-  string value;
-};
-
 struct URLRequest {
   // The URL to load.
   string url;
 
   // The HTTP method if applicable.
   string method = "GET";
 
   // Additional HTTP request headers.
   array<HTTPHeader>? headers;
 
@@ skipping 72 matching lines @@
   Start(URLRequest request) => (URLResponse response);
 
   // If the request passed to |Start| had |auto_follow_redirects| set to false,
   // then upon receiving an URLResponse with a non-NULL |redirect_url| field,
   // |FollowRedirect| may be called to load the URL indicated by the redirect.
   FollowRedirect() => (URLResponse response);
 
   // Query status about the URLLoader.
   QueryStatus() => (URLLoaderStatus status);
 };