[New multicol] Make -webkit-column-break-inside:avoid work.

adjustForUnsplittableChild() simply forgot to check whether we
were inside flow thread based multicol, which kept it from even
considering what -webkit-column-break-inside was set to in such cases.

BUG=346661

Review URL: https://codereview.chromium.org/179763003

git-svn-id: svn://svn.chromium.org/blink/trunk@167882 bbb929c8-8fbe-4397-9dbb-9b2b20218538