Stretch nested columns to the block-size of the outer fragmentainer.

This is required by the spec.
https://www.w3.org/TR/css-break-3/#box-splitting "When a box breaks, its
content box extends to fill any remaining fragmentainer extent"

We can finally DCHECK that minimal_space_shortage is set, rather than
bailing if it isn't.

Add a test that demonstrates that this is necessary in order to paint
the backgrounds and column rules correctly. It also fixes a problem with
offsetLeft and offsetTop, because we confused the legacy code (still
used for offsetFoo) when writing back fragmentainer groups that didn't
take up all available space.

Bug: 829028
Change-Id: Ic23f94d2b9db4d4cc4f8d9036b32676a933f82c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3144154
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/main@{#919172}