Replace object return value by class type

Summary:
This code was 7.2 compatible, but not 7.1, without real benefit,
as it used https://wiki.php.net/rfc/object-typehint.

Fixes T1335.

Test Plan: Test under PHP 7.1.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1335

Differential Revision: https://devcentral.nasqueron.org/D1308