Provide viewer to CustomFields in ApplicationSearch

Summary: Fixes T4663.

Test Plan:
Added a custom field which accesses the viewer's username.

{F133249}

Reviewers: carl, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4663

Differential Revision: https://secure.phabricator.com/D8595

src/applications/search/engine/PhabricatorApplicationSearchEngine.php

@@ -511,6 +511,7 @@ abstract class PhabricatorApplicationSearchEngine {
         $fields = PhabricatorCustomField::getObjectFields(
           $object,
           PhabricatorCustomField::ROLE_APPLICATIONSEARCH);
+        $fields->setViewer($this->requireViewer());
       } else {
         $fields = null;
       }