code: func (c *contextAwareUrlFetch) RoundTrip(r *http.Request) (*http.Response, error) {
  	ctx := r.Context()
  	// We assume context.Background() always returns exact same object.
  	if ctx == background {
  		ctx = c.c
  	}
  	return urlfetch.Get(ctx).RoundTrip(r)
  }
docs: // RoundTrip is part of http.RoundTripper interface.