Update APIs to use new Google cloud paths.

common/gcloud/gs/writer.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 gs
 
 import (
         "io"
         "time"
 
+        gs "cloud.google.com/go/storage"
         "github.com/luci/luci-go/common/errors"
         log "github.com/luci/luci-go/common/logging"
         "github.com/luci/luci-go/common/retry"
         "golang.org/x/net/context"
-        gs "google.golang.org/cloud/storage"
 )
 
 // Writer is an augmented io.WriteCloser instance.
 type Writer interface {
         io.WriteCloser
 
         // Count returns the number of bytes written by the object.
         Count() int64
 }
 
@@ skipping 48 matching lines @@
                                 "delay":      d,
                                 "bucket":     w.bucket,
                                 "path":       w.relpath,
                         }.Warningf(w, "Transient error closing GS Writer. Retrying...")
                 })
 }
 
 func (w *prodWriter) Count() int64 {
         return w.count
 }