merge_pr_48580

Add implicit anchor relationship for popover invokers

When PopoverAnchorRelationships is enabled, include popover invokers
as "implicit anchor elements" for anchor positioning.

This also adds a test of the behavior when nesting within other top layer elements, and vice versa. Bug: 364669918 Change-Id: Ie1d362a41e3aec72d62dbfe378e65ea9a262c324 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5846489 Auto-Submit: Mason Freed masonf@chromium.org Commit-Queue: Joey Arhar jarhar@chromium.org Reviewed-by: Joey Arhar jarhar@chromium.org Cr-Commit-Position: refs/heads/main@{#1367631}