Bug 1779645 Part 1 - Switch CSS named page fragmentation to happen at reflow...

Bug 1779645 Part 1 - Switch CSS named page fragmentation to happen at reflow instead of frame construction r=dholbert

This also adds a small post-processing step for frame-construction to ensure
that replaced frames (or more generally frames with content but no children)
have the auto page-name set on them.

When page-name value tracking is switched to be lazy, this post-processing step
will likely be redundant and can be removed.

Differential Revision: https://phabricator.services.mozilla.com/D152701