endpoints: add GitLab DeviceAuthURL

See GitLab documentation https://docs.gitlab.com/ee/api/oauth2.html#device-authorization-grant-flow.

Change-Id: Ideffbfcb67e0b25251a0b18148e57eab43124d37
GitHub-Last-Rev: 6ecd45efff1a39fa7f31da96be70243b6029d16f
GitHub-Pull-Request: golang/oauth2#733
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/600095
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matt Hickford <matt.hickford@gmail.com>
Reviewed-by: Matt Hickford <matt.hickford@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Bypass: Matt Hickford <matt.hickford@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

endpoints/endpoints.go

diff --git a/endpoints/endpoints.go b/endpoints/endpoints.go
index 89ed30f..950754c 100644
--- a/endpoints/endpoints.go
+++ b/endpoints/endpoints.go
@@ -62,8 +62,9 @@
 
 // GitLab is the endpoint for GitLab.
 var GitLab = oauth2.Endpoint{
-	AuthURL:  "https://gitlab.com/oauth/authorize",
-	TokenURL: "https://gitlab.com/oauth/token",
+	AuthURL:       "https://gitlab.com/oauth/authorize",
+	TokenURL:      "https://gitlab.com/oauth/token",
+	DeviceAuthURL: "https://gitlab.com/oauth/authorize_device",
 }
 
 // Google is the endpoint for Google.

gitlab/gitlab.go

diff --git a/gitlab/gitlab.go b/gitlab/gitlab.go
index 1231d75..aea2dfe 100644
--- a/gitlab/gitlab.go
+++ b/gitlab/gitlab.go
@@ -6,11 +6,8 @@
 package gitlab // import "golang.org/x/oauth2/gitlab"
 
 import (
-	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/endpoints"
 )
 
 // Endpoint is GitLab's OAuth 2.0 endpoint.
-var Endpoint = oauth2.Endpoint{
-	AuthURL:  "https://gitlab.com/oauth/authorize",
-	TokenURL: "https://gitlab.com/oauth/token",
-}
+var Endpoint = endpoints.GitLab