[LayoutNG] fix minor painting bug

The fix initializes skip_background argument to PaintNormalBoxShadow
to match Legacy.

Bug: 591099
Change-Id: I180dc74f5a6478c7b56f69d25e3c0a94e88bbc65
Reviewed-on: https://chromium-review.googlesource.com/c/1401265
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Aleks Totic <atotic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620869}
2 files changed