ColumnBalancer: don't skip bounds checking on first or last fragmentainer groups.

There should be no need for this special-code. Whatever manages to end up
before the first column or after the last one is totally uninteresting to the
column balancer.

Review URL: https://codereview.chromium.org/1886703002

Cr-Commit-Position: refs/heads/master@{#387150}
1 file changed