[LayoutNG] Cleanup computing margins/border/padding in NGLineBreaker

1. Padding for atomic inline isn't used. Removed computing it.
2. Both converting to NGBoxStrut and then to NGLineBoxStrut
   should use the same writing-mode. Added ng_length_utils
   functions for the two conversions to occur at the same
   place.

Bug: 636993
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I3deace37ae5334af0f919d4c557effe067328da9
Reviewed-on: https://chromium-review.googlesource.com/1181004
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586218}
6 files changed