Introduce authentication service.

services/authentication/authentication.mojom

Index: services/authentication/authentication.mojom
diff --git a/services/authentication/authentication.mojom b/services/authentication/authentication.mojom
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+// 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.
+
+[JavaPackage="org.chromium.mojo.authentication"]
+module authentication;
+
+// Interface to handle user identity and authentication tokens.
+// TODO(qsr): This API only handles google accounts at this time. It will need
+//            to be extended to allow generic account manager on the platform.
+interface AuthenticationService {
+  // Requests a Google account to use. In case of success, error will be null.
+  // In case of error, username will be null and error will contain a
+  // description of the error.
+  SelectAccount() => (string? username, string? error);
+
+  // Requests an oauth2 token for the given Google account with the given
+  // scopes.  In case of error, username will be null and error will contain a
+  // description of the error.
+  GetOAuth2Token(string username, array<string> scopes) =>
+      (string? token, string? error);
+
+  // Requests to clear a previously acquired token. This should be called when a
+  // token is refused by a server component before requesting a new token to
+  // clear the token from any cache.
+  ClearOAuth2Token(string token);
+};