Remove CHECK that can get hit due to foster parenting

In this case:

{{#}}

the prior CHECK could get hit. Since we're just popping the part
root, this CL just removes the CHECK.

Note that the prior CL that added the test for this inadvertently
made the test not work, due to the name. See crbug.com/379764806.

Bug: 40271855
Bug: 379764806
Change-Id: Ib723cf166ecbe0f2a7da855c27d56e84e2957fba
Fixed: 372994343
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6033772
Reviewed-by: Di Zhang dizhangg@chromium.org
Commit-Queue: Di Zhang dizhangg@chromium.org
Auto-Submit: Mason Freed masonf@chromium.org
Cr-Commit-Position: refs/heads/main@{#1385341}