R1:cb2f7106069b

Provide the document content as a context object when rendering Phriction documents

Summary: Ref T13077. The context object wasn't being passed into the engine properly here, affecting relative link rendering in Phriction.

Test Plan: Viewed rendered Phriction documents with relative links, got clean renders.

Maniphest Tasks: T13077

Differential Revision: https://secure.phabricator.com/D19115