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.

The first version of this patch had a security bug whereby -webkit-boxes were
upcast incorrectly to RenderBlockFlows. This was to preserve broken behavior
where we allowed -webkit-boxes to claim to be multi-column blocks, which
doesn't actually work. This patch now disables that broken behavior.

Also changing an old security bug effected by this to dumpAsText.

BUG=302024

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

git-svn-id: svn://svn.chromium.org/blink/trunk@163324 bbb929c8-8fbe-4397-9dbb-9b2b20218538