LayoutBlock::addChild() can destroy its own object and that of any out-of-flow
or floating siblings - for that reason it's not safe to access the object or any
of its siblings after calling it. So when checking for an empty anonymous block
here, do so after removing its child and before we add it to its new home.

The check was introduced in wkbug.com/78250 and a follow-up wkbug.com/79681
so we have test coverage from those two CLs as well.

BUG=598722

Review URL: https://codereview.chromium.org/1843213002

Cr-Commit-Position: refs/heads/master@{#385009}