Add simplest service/urlfetch implementation.

impl/prod/urlfetch.go

Index: impl/prod/urlfetch.go
diff --git a/impl/prod/urlfetch.go b/impl/prod/urlfetch.go
new file mode 100644
index 0000000000000000000000000000000000000000..14c4bec14e931b0716f1596ef5e3027f4e9a55ac
--- /dev/null
+++ b/impl/prod/urlfetch.go
@@ -0,0 +1,20 @@
+// 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}
+	})
+}