Resurrect ASSERT(isFirstAfterBreak(flowThreadOffset()) || !box.paginationStrut()).

This assertion effectively became disabled by accident by
https://codereview.chromium.org/1856373002 , so that fuzzer bug 551312 stopped
asserting, although that bug is still very much present.

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/1876713003

Cr-Commit-Position: refs/heads/master@{#386418}
1 file changed