Fix painting order for floats with shape-outside

When removing the old positioning behavior, the code that makes a float
with shape-outside gain a layer was left behind. This patch removes
that, so now floats with shape-outside are painted just like floats
without shape-outside.  Also remove the isFloatingWithShapeOutside
helper method because its use is now somewhat redundant, and the check
it does is now only needed in one place.

BUG=286448

Review URL: https://chromiumcodereview.appspot.com/23866007

git-svn-id: svn://svn.chromium.org/blink/trunk@157396 bbb929c8-8fbe-4397-9dbb-9b2b20218538