Extend LUCI_CONTEXT["local_auth"] protocol to understand accounts.

common/auth/localauth/rpcs/getoauthtoken.go

 // Copyright 2017 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package rpcs
 
 import (
-        "fmt"
+        "github.com/luci/luci-go/common/errors"
 )
 
 // GetOAuthTokenRequest is parameters for GetOAuthToken RPC call.
 type GetOAuthTokenRequest struct {
-        Scopes []string `json:"scopes"`
-        Secret []byte   `json:"secret"`
+        Scopes    []string `json:"scopes"`
+        Secret    []byte   `json:"secret"`
+        AccountID string   `json:"account_id"`
 }
 
-// Validate checks that scopes and secret are set.
+// Validate checks that the request is structurally valid.
 func (r *GetOAuthTokenRequest) Validate() error {
         switch {
         case len(r.Scopes) == 0:
-                return fmt.Errorf(`Field "scopes" is required.`)
+                return errors.New(`field "scopes" is required`)
         case len(r.Secret) == 0:
-                return fmt.Errorf(`Field "secret" is required.`)
+                return errors.New(`field "secret" is required`)
         }
+        // TODO(vadimsh): Uncomment when all old auth server are turned off.
+        //case r.AccountID == "":
+        //      return errors.New(`field "account_id" is required`)
+        //}
         return nil
 }
 
 // GetOAuthTokenResponse is returned by GetOAuthToken RPC call.
 type GetOAuthTokenResponse struct {
         BaseResponse
 
         AccessToken string `json:"access_token"`
         Expiry      int64  `json:"expiry"`
 }