[LayoutNG] Adjust BoxInfo::item_index when items are removed

When NGInlineItemsBuilder removed an item, it needs to adjust
BoxInfo::item_index. This patch fixes it.

This was revealed in crrev.com/c/1333588, tests will fail
with the CL without this fix.

Bug: 636993
Change-Id: I7a996e358c0daa8e6c7aa1b5227e4027fe0295bd
Reviewed-on: https://chromium-review.googlesource.com/c/1337210
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608266}
1 file changed