[LayoutNG] Rebaseline improvements in absolute/fixed positioning

external/wpt/css/cssom/getComputedStyle-insets-absolute.html
  Legacy passes 180 while NG passes 192.

external/wpt/css/cssom/getComputedStyle-insets-fixed.html
  Legasy passes 168 while NG passes 180.

Based on build 9840.
https://test-results.appspot.com/data/layout_results/linux_layout_tests_layout_ng/9840/layout-test-results/results.html

TBR=atotic@chromium.org, ikilpatrick@chromium.org, mstensho@chromium.org
NOTRY=true

Bug: 591099
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I54b8405ac3fc2a03af399a5e842b3560ce80f8ed
Reviewed-on: https://chromium-review.googlesource.com/1233073
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592288}
3 files changed