Make appengine connection explicitly owned by impl/prod.

impl/prod/urlfetch.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 prod
 
 import (
         "net/http"
 
         uf "github.com/luci/gae/service/urlfetch"
         "golang.org/x/net/context"
         "google.golang.org/appengine/urlfetch"
 )
 
 // useURLFetch adds a http.RoundTripper implementation to the context.
 func useURLFetch(c context.Context) context.Context {
         return uf.SetFactory(c, func(ci context.Context) http.RoundTripper {
-                return &urlfetch.Transport{Context: ci}
+                return &urlfetch.Transport{Context: AEContext(ci)}
         })
 }