Tighten up anonymous block creation for RenderBlockFlow

Anonymous blocks and anonymous multi-column blocks are RenderBlockFlows,
not RenderBlock. This change moves some code around and tightens up
types where applicable.

BUG=302024

Review URL: https://codereview.chromium.org/98263002

git-svn-id: svn://svn.chromium.org/blink/trunk@163054 bbb929c8-8fbe-4397-9dbb-9b2b20218538