[New multicol] Eliminate the need for RenderMultiColumnBlock.

Having a distinct renderer for multicol is problematic, both
table cells and list items may become multicol containers. Also,
if we want to use the new multicol implementation to do
pagination, we need RenderView to be able to do multicol as well.

Moved most of the logic over to RenderMultiColumnFlowThread, but some
hooks are needed in RenderBlockFlow as well.

This is based on work by hyatt@apple.com in WebKit. Original patches:
https://bugs.webkit.org/show_bug.cgi?id=127365
https://bugs.webkit.org/show_bug.cgi?id=127565

Added Layout tests for multicol list-item and table-cell.

Also added tests for turning a regular block into multicol, and back into a
regular block (passed before, passes now).

BUG=none

Review URL: https://codereview.chromium.org/179993006

git-svn-id: svn://svn.chromium.org/blink/trunk@167891 bbb929c8-8fbe-4397-9dbb-9b2b20218538