Floats Don't Overhang Their Block Formatting Context
Per http://www.w3.org/TR/CSS2/visuren.html#float-position: "References
to [the elements] in the [rules that govern the behaviour of floats]
refer only to other elements in the same block formatting context
as the float."

This means we don't need to look for intruding or overhanging floats
in boxes that establish a new block formatting context. I couldn't
find a test where we failed this rule but we do appear to be doing
unnecessary work in rebuildFloatsFromIntruding() so make sure we
avoid adding floats from different block formatting contexts.

The next step here is to merge the appropriate parts of avoidFloats()
and createsBlockFormattingContext() as they are frequently synonymous.

BUG=

Review URL: https://codereview.chromium.org/237803002

git-svn-id: svn://svn.chromium.org/blink/trunk@172530 bbb929c8-8fbe-4397-9dbb-9b2b20218538