Change AuthenticatingURLLoader to be a URLLoaderInterceptor

mojo/services/authenticating_url_loader/public/interfaces/authenticating_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 "mojo/public/interfaces/network/url_request.mojom";
-import "mojo/public/interfaces/network/url_response.mojom";
-
-interface AuthenticatingURLLoader {
-  // Loads the given |request|, asynchronously producing |response|. If the
-  // server indicates that authentication is needed, attempts to obtain an
-  // OAuth2 token from the user and use that as authentication. Consult
-  // |response| to determine if the request resulted in an error, was
-  // redirected, or has a response body to be consumed.
-  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);
-};