Refactor avoidsFloats() to reflect CSS2.1 spec more clearly

Introduce the 'createsBlockFormatting' concept to RenderBlock::avoidsFloats()
and remove a couple of now-redundant virtualizations of the function.

BUG=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=173549

Review URL: https://codereview.chromium.org/268833003

git-svn-id: svn://svn.chromium.org/blink/trunk@176557 bbb929c8-8fbe-4397-9dbb-9b2b20218538