Remove DCHECK in column balancer that failed because of flexbox bugs.

This DCHECK was useful (detects broken layout, but nothing more dangerous than
that). However, as long as we don't paginate flex items at their final block
position (see bug 606350), we cannot assert like this, because it's going to
fail under certain circumstances.

BUG=685047

Review-Url: https://codereview.chromium.org/2797313003
Cr-Commit-Position: refs/heads/master@{#462447}
2 files changed