Revert 163054 "Tighten up anonymous block creation for RenderBlo..."

Caused bad casts on ClusterFuzz.

> 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

TBR=leviw@chromium.org

Review URL: https://codereview.chromium.org/105203005

git-svn-id: svn://svn.chromium.org/blink/trunk@163192 bbb929c8-8fbe-4397-9dbb-9b2b20218538