Add UseBackground.

impl/prod/context_vm.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.
+
+// +build !appengine
+
+package prod
+
+import (
+        "golang.org/x/net/context"
+        "google.golang.org/appengine"
+)
+
+// UseBackground is the same as Use except that it activates production
+// implementations which aren't associated with any particular request.
+//
+// This is only available on Managed VMs.
+func UseBackground(c context.Context) context.Context {
+        return setupAECtx(c, appengine.BackgroundContext())
+}