There's no call to AddRefreshObserver(FlushType::Layout), so we don't
need to track layout flushes. The only reason we need
mLayoutFlushObservers is so that reflows triggered from the style flush
get processed. But at that point, we can just flush layout directly.
There's no really good distinction between style flushes and layout
flushes with container queries anyways, so this makes the code simpler
to reason about.
Differential Revision: https://phabricator.services.mozilla.com/D209924
57df8917ac