impl/prod: Embed AppEngine SDK into Context.

impl/prod/context_vm.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.
 
 // +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())
+//
+// In addition, UseBackground installs Google AppEngine SDK Context values into
+// the Context. See the warning in Use for more information on pitfalls related
+// to usage of the raw AppEngine SDKs alongside luci/gae.
+func UseBackground() context.Context {
+        return setupAECtx(appengine.BackgroundContext())
 }