[css-anchor-position] Test visibility with fixed anchored element

This patch adds an additional testcase for https://crrev.com/1283191
that tests the case of a fixed-position anchored element with
position-visibility: anchors-visible. This is interesting because it
requires correctly handling the view's scroll.

Bug: 329703412
Change-Id: Ib6043d285633932966e643b2c3bcd0e7a6ab9628
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5429056


Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Auto-Submit: Philip Rogers <pdr@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283378}