[css-tables] Don't use internal layout structure when iterating all rows

Instead use basic firstChild and nextSibling. This avoids a crash when
the internal layout structure is dirty.

Add DCHECK to discourage future mistakes.

BUG=627839

Review-Url: https://codereview.chromium.org/2146163005
Cr-Commit-Position: refs/heads/master@{#405555}
4 files changed