Fix moveBefore() DHECK failure
FlatTreeParentChanged() was originally intended for slot re-assignments.
The DCHECK for nodes being slottable failed for moveBefore() for comment
nodes, for instance. Separate out the common FlatTreeParentChanged()
method and add DCHECKs to a new ParentSlotChanged() instead.
Bug: 338071841
Change-Id: I18dd20e000c7d1ef9cdf6f5bb4034dd651839778
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5503469
Reviewed-by: Dominic Farolino dom@chromium.org
Commit-Queue: Rune Lillesveen futhark@chromium.org
Cr-Commit-Position: refs/heads/main@{#1295003}