[LayoutNG] Mark first line box dirty only when it exists

When marking the first line box dirty, this patch checks
if the line box exists.

Normally NGBlockLayoutAlgorithm suppresses box fragment
if no line boxes exist. However, NGColumnLayoutAlgorithm
may generate box fragments for column boxes.

Bug: 636993
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ie5697a524cd6fc1067fc4c03466e88fa8d5da68b
Reviewed-on: https://chromium-review.googlesource.com/c/1288769
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600969}
1 file changed