More chat scrolling fixes.

Ensure there's always a small "last child" of the chat messages <div>.
This fixes scrolling when the last message was very large.

Re-invoke maybeScrollMessagesView() on a self-scroll; this ensures that
delayed relayout will still scroll to the bottom after it completes.

Change-Id: I60fecc70b1dec7c135441a7135090615d0a4cf42