This rewrites scrollbar layout to work with regular reflow rather than
box layout.
Overall it's about the same amount of code (mostly because
nsScrollbarFrame::Reflow is sorta hand-rolled), but it cleans up a bit
and it is progress towards removing XUL layout altogether, without
getting into much deeper refactoring.
This also blocks some other performance improvements and refactorings I
want to make in this code.
We make some assumptions to simplify the code that to some extent were
made already before, both explicitly and by virtue of using XUL layout.
In particular, we assume that scrollbar / slider / thumb has no border or
padding and that the writing-mode is horizontal ltr.
Differential Revision: https://phabricator.services.mozilla.com/D173489
38b10eafda