Bug 1839486 [wpt PR 40651] - Fix external/wpt/css/css-position/fixed-z-index-blend...

Bug 1839486 [wpt PR 40651] - Fix external/wpt/css/css-position/fixed-z-index-blend.html, a=testonly

Automatic update from web-platform-tests
Fix external/wpt/css/css-position/fixed-z-index-blend.html

1. Flakiness caused by missing reftest-wait in the reference.
   Remove requestAnimationFrame from the reference so that it no
   longer needs reftest-wait.
2. Change scrollBy amount from 3000 to 4000 to scroll the target
   green rect into view.

Bug: 1429309
Change-Id: I7756e6f163d76f8eeeacd4d72c6c6196d3b921d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4628850
Reviewed-by: Mason Freed <masonf@chromium.org>
Auto-Submit: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1160938}

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wpt-commits: 2ac776fb32d8f4cfe08a41773dd4adfc6750baf6
wpt-pr: 40651