Provide backward compatibility for callback functions in
GuzzleHttpRequest, which was missing in T202110, and restore
GuzzleHttpRequest as the default provided by HttpRequestFactory.
Bug: T212175
Depends-On: I4b45e79d35252d13f714f3271b87301ca515121a
Change-Id: I60d1a034b44874f6d24a04058db264eeb565f5e1