Use native Pub/Sub library primitives.

common/gcloud/pubsub/scopes.go

+// 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.
+
+package pubsub
+
+import (
+        "google.golang.org/cloud/pubsub"
+)
+
+var (
+        // PublisherScopes is the set of OAuth2 scopes needed for a publisher to
+        // publish messages.
+        PublisherScopes = []string{
+                pubsub.ScopePubSub,
+        }
+
+        // SubscriberScopes is the set of OAuth2 scopes needed for a subscriber to
+        // pull and acknowledge messages.
+        SubscriberScopes = []string{
+                pubsub.ScopePubSub,
+        }
+)