[LayoutNG] Downgrade unsafe access to NGOffsetMapping into a CHECK

We currently have some unsafe access to NGOffsetMapping while the block
was missed in previous layout. This patch blocks such access by adding
a CHECK.

We should fix the root cause later.

Bug: 962129
Change-Id: Ia661f29a8749a030a87658fb6060146c1a5baea0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1620773
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Auto-Submit: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661545}
1 file changed