Bug 1749932 [wpt PR 32360] - Include will-change:transform as transform for backgroun...

Bug 1749932 [wpt PR 32360] - Include will-change:transform as transform for background-attachment:fixed, a=testonly

Automatic update from web-platform-tests
Include will-change:transform as transform for background-attachment:fixed

This is to prevent jump of background when an element with will-change:
transform changes transform from initial to non-identity. This is not in
the spec yet, so the test case is tentative with a css-transforms-1 bug
is filed at https://github.com/w3c/csswg-drafts/issues/6951. Firefox
already has this behavior.

Bug: 1286593
Change-Id: I4247824b5af28613644bcd334832ff22b9483384
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3385337
Reviewed-by: David Baron <dbaron@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#959058}

--

wpt-commits: a04b6be3ebaeea3fd79b82e9447ba1e0b864babe
wpt-pr: 32360