Multi-col and column-spanning elements create new block formatting contexts

Multi-col and column-span elements create new block formatting contexts
so their margins should not collapse together if they are self-collapsing blocks.

http://www.w3.org/TR/css3-multicol/#the-multi-column-model: "A multi-column element establishes a new block formatting context, as per CSS 2.1 section 9.4.1."

http://www.w3.org/TR/css3-multicol/#column-span: "
The element establishes a new block formatting context."

BUG=

Review URL: https://codereview.chromium.org/147643002

git-svn-id: svn://svn.chromium.org/blink/trunk@166855 bbb929c8-8fbe-4397-9dbb-9b2b20218538