Move containsFloats to RenderBlockFlow.

This forces callers which are interested in floats to
first ensure that they are looking at a block flow.

BUG=302024

Review URL: https://codereview.chromium.org/400383004

git-svn-id: svn://svn.chromium.org/blink/trunk@178856 bbb929c8-8fbe-4397-9dbb-9b2b20218538