[LayoutNG] Make fragmenting floats use util method for ignoring block-start margins.

This is just a cleanup and should have no behavioral change.

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: If69feb08df2f5247e09654d7ae0d7d1201a2d47a
Bug: 635619
Reviewed-on: https://chromium-review.googlesource.com/1150623
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578914}
1 file changed