Add back [popover][anchor] CSS rule, behind the flag
The [1] CL removed this, but never added it back (due to how much
work that used to take). Now that @supports blink-feature is a
thing, this puts back the CSS.
[1] https://chromium-review.googlesource.com/c/chromium/src/+/5476428
Bug: 335223786
Change-Id: Idc37e3c5f783259bda3bcb73e1e427233000c46c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5555622
Auto-Submit: Mason Freed masonf@chromium.org
Reviewed-by: Joey Arhar jarhar@chromium.org
Commit-Queue: Mason Freed masonf@chromium.org
Cr-Commit-Position: refs/heads/main@{#1304493}