[New Multicolumn] Destroy line box tree when evacuating the flowthread for multicol.

The life of (in)line boxes is mysterious. They are allowed to
temporarily point to dead inline children or line box parents. It's
also allowed for InlineTextBox objects to be unrooted temporarily,
apparently. All this should be cleaned up, but for now, just do the
same in the new multicol code as everywhere else: manually take care
of deleting the line box tree before destroying the box you're
emptying.

BUG=362865

Review URL: https://codereview.chromium.org/236913002

git-svn-id: svn://svn.chromium.org/blink/trunk@172054 bbb929c8-8fbe-4397-9dbb-9b2b20218538