Add support for the column-fill property.

Note: This is only supported in the (new) region based multicol implementation.

With column-fill support, a lot of multicol tests need an update.
The old implementation behaved as if column-fill were 'auto', but the
initial value is 'balance', so now we need to be explicit about that.
For auto-height tests it doesn't really matter - such multicols are always
balanced anyway.

Already reviewed in WebKit by David Hyatt.
See https://bugs.webkit.org/show_bug.cgi?id=117693

BUG=302897

Review URL: https://codereview.chromium.org/25687002

git-svn-id: svn://svn.chromium.org/blink/trunk@159227 bbb929c8-8fbe-4397-9dbb-9b2b20218538