This is the second of two big chunks of code moved into the new
RestyleManager class from another sources.
Note that the undisplayed map remains in nsFrameManager, although it
could perhaps have moved.
This moves restyling management out of nsCSSFrameConstructor (thus
reducing its size), and keeps the restyling code closer together.
This is the first of two big chunks of code moved in this patch series.
A later patch in this series will move related code from nsFrameManager
into the same destination file.
