Add UseBackground.

impl/prod/context.go

Index: impl/prod/context.go
diff --git a/impl/prod/context.go b/impl/prod/context.go
index 8a613253d2aa4cd85cc42a80895cc36ee07d1da8..007d006fb2a3ca736f9a762b2a26f7228590907f 100644
--- a/impl/prod/context.go
+++ b/impl/prod/context.go
@@ -86,6 +86,12 @@ func Use(c context.Context, r *http.Request) context.Context {
 	return setupAECtx(c, appengine.NewContext(r))
 }
 
+// UseBackground is the same as Use except that it activates production
+// implementations which aren't associated with any particular request.

[Vadim Sh., 2016/01/25 18:56:17]

What are conditions of it working? Can you please document them (i.e. MVM or
backgroud thread).

+func UseBackground(c context.Context) context.Context {
+	return setupAECtx(c, appengine.BackgroundContext())
+}
+
 // UseRemote is the same as Use, except that it lets you attach a context to
 // a remote host using the Remote API feature. See the docs for the
 // prerequisites.