Bug 307884

Fixing bug 291864 ([css-anchor-position-1] position-area for scrollable containing blocks should use entire scrollable area, not just the initial scrollport bounds) requires us to know the scrollable content area bounds before we do abspos layout. Unfortunately this is currently computed together with the overflow of out-of-flow boxes after both in-flow and out-of-flow layout is completed, so we need to split apart that method so that we can get the in-flow overflow computed before out-of-flow layout, and then update to include out-of-flow overflow once out-of-flow layout is completed. Since this is essentially a no-op refactor, we're filing it as an independent PR to make it easier to review and test.