R1:a09b298d85d7

Correct DOM node metadata to let inline comments work against block-based diffs

Summary: Depends on D20833. Ref T13425. This look like it "just works"?

Test Plan: Left inline comments on a Juptyer notebook. Nothing seemed broken? Confusing.

Maniphest Tasks: T13425

Differential Revision: https://secure.phabricator.com/D20834