Declare missing class property in PhabricatorTimelineEngine

Summary: setRequest() and getRequest in this class access the undefined property $request. Thus define this property.

Test Plan: Run PHPStan static code analysis; grep the code in this class.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25716