Handle percent height children of multicol containers correctly.

The render flow thread block that's reparented between the multicol container
and its actual children is just an implementation detail, invisible to DOM and CSS.
Its height will always be auto, and we need to bypass it.

BUG=351657

Review URL: https://codereview.chromium.org/197113002

git-svn-id: svn://svn.chromium.org/blink/trunk@169050 bbb929c8-8fbe-4397-9dbb-9b2b20218538