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