ruby: Propagate block-end annotation space in multicol containers

Previously, block-end annotation space propagation was disabled for
children of balanced multicol containers to avoid issues in an existing
WPT test about column balancing. This CL enables it.

This CL fixes BlockLayoutAlgorithm::FinishLayout() so that to compute
the available annotation space correctly for fragmented containers.

Bug: 445727139, 538260158
Change-Id: I27848eba96cb7f8ae8eb775d725455e0bc8d8748
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8294040
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1691451}