[LayoutNG] Fix dialog element positioning when it is out-of-flow

LayoutBlockFlow::UpdateBlockLayout() has a specialized
positioning for HTML <dialog> element when it is out-of-flow.

This patch implements the logic by sharing the code in
LayoutBlockFlow.

Bug: 635619
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I16870f926dc0ad6fbb6bee4787f3f6d5e30b31c7
Reviewed-on: https://chromium-review.googlesource.com/1108240
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Aleks Totic <atotic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569195}
4 files changed