merge_pr_49846

DOM: Add basic moveBefore() MutationObserver tests

See whatwg/dom#1307 (comment).

R=[email protected]

Bug: 40150299
Change-Id: I6be9d91d4e20690694fb20480f0d1d13a766b117
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6108064
Reviewed-by: Noam Rosenthal [email protected]
Commit-Queue: Dominic Farolino [email protected]
Cr-Commit-Position: refs/heads/main@{#1400321}