[LayoutNG] UpdateMargins is no longer needed by LayoutNGBlockFlow

After CL 827548, margins are always copied to Legacy.
Additional copy in LayoutNGBlockFlow is no longer needed.
Ian says so.

Bug: 740993
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: If1aee4b15a966d58bd62f66190fdf00f035ed0b4
Reviewed-on: https://chromium-review.googlesource.com/830474
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Aleks Totic <atotic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524538}
3 files changed