[LayoutNG] More "interesting" behaviour with empty blocks clearance.

This is one addition to the empty block clearance behaviour added
previously. If we've been affected by clearance, we need to reset the
incoming margin strut and perform a re-layout in order to get the correct
end margin strut for positioning.

This is because the incoming margin strut doesn't "touch" the empty block
anymore.

Bug: 635619
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: Ied3528672bb6544c047691d0c556c2f09497f52c
Reviewed-on: https://chromium-review.googlesource.com/582238
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492599}
3 files changed