merge_pr_50911

Remove stale TODO about isolation nodes and fragmentation.

Fragmented isolation nodes have been allowed ever since LayoutNG block
fragmentation got introduced. Include a test that should have failed if
this wasn't implemented correctly.

The code that checked for fragmentation got removed here:
https://chromium-review.googlesource.com/c/chromium/src/+/4804328

Bug: 40596162
Change-Id: Ia6a52298ca107528a0c17c4c82b933065505cbef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6298808
Commit-Queue: Morten Stenshorne mstensho@chromium.org
Reviewed-by: Xianzhu Wang wangxianzhu@chromium.org
Reviewed-by: Vladimir Levin vmpstr@chromium.org
Cr-Commit-Position: refs/heads/main@{#1424102}