A column-span:all element should always establish a new formatting context.

This also applies when the element isn't contained by a multicol container
(which means that we shouldn't require the spanner placeholder to be present).

BUG=709362

Review-Url: https://codereview.chromium.org/2799363003
Cr-Commit-Position: refs/heads/master@{#462865}
3 files changed