Avoid unnecessary extra calls to  RenderBlockFlow::addIntrudingFloats()

The first inflow block in a container that has a leading float will call
addIntrudingFloats twice. Adjust the logic to avoid this unnecessary extra
call.

Review URL: https://codereview.chromium.org/235933006

git-svn-id: svn://svn.chromium.org/blink/trunk@172156 bbb929c8-8fbe-4397-9dbb-9b2b20218538