[layout] Fix null-deref when attempting to mark a scrollbar for layout
Custom scrollbar parts are weird in that they don't have a parent.
We'd unconditionally attempt to access their containing block
causing a null-deref.
Fixed: 355016913
Change-Id: I9c32cf7efc00543021111bb3eaa1e8488052264f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5760674
Reviewed-by: David Grogan dgrogan@chromium.org
Commit-Queue: Ian Kilpatrick ikilpatrick@chromium.org
Cr-Commit-Position: refs/heads/main@{#1337141}