Update APIs to use new Google cloud paths.

common/auth/internal/gce.go

 // Copyright 2015 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 internal
 
 import (
+        "cloud.google.com/go/compute/metadata"
         "golang.org/x/net/context"
         "golang.org/x/oauth2"
         "golang.org/x/oauth2/google"
-        "google.golang.org/cloud/compute/metadata"
 
         "github.com/luci/luci-go/common/data/stringset"
         "github.com/luci/luci-go/common/logging"
 )
 
 type gceTokenProvider struct {
         account string
 }
 
 // NewGCETokenProvider returns TokenProvider that knows how to use GCE metadata server.
@@ skipping 23 matching lines @@
 
 func (p *gceTokenProvider) MintToken() (*oauth2.Token, error) {
         src := google.ComputeTokenSource(p.account)
         return src.Token()
 }
 
 func (p *gceTokenProvider) RefreshToken(*oauth2.Token) (*oauth2.Token, error) {
         // Minting and refreshing on GCE is the same thing: a call to metadata server.
         return p.MintToken()
 }