Enable repaint after layout when positioning new floats.

Currently the RenderBlockFlow::positionNewFloats class will paint the float
immediately. This CL updates the code to allow us to paint later if
repaintAfterLayout is enabled.

BUG=310398

Review URL: https://codereview.chromium.org/98383005

git-svn-id: svn://svn.chromium.org/blink/trunk@164057 bbb929c8-8fbe-4397-9dbb-9b2b20218538