Make MultiColumnFragmentainerGroup::m_columnSet const.

Ideally, I'd like to get rid of the member altogether, but that would require a
lot of refactoring.

This is a preparatory patch for a fix for bug 604609.

BUG=604609

Review URL: https://codereview.chromium.org/1898293003

Cr-Commit-Position: refs/heads/master@{#388326}
3 files changed