[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

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