Bug 1735865 [wpt PR 31248] - Highlight API: Don't paint text-shadow for custom :...

Bug 1735865 [wpt PR 31248] - Highlight API: Don't paint text-shadow for custom ::highlight pseudos, a=testonly

Automatic update from web-platform-tests
Highlight API: Don't paint text-shadow for custom ::highlight pseudos

There are some uncertanties [1] [2] [3] about how text-shadow should
work for highlight pseudos. Until these are resolved, disable
text-shadow for custom ::highlight so that when custom Highlight API
ships, developers don't take dependencies on text-shadow behavior that
may change or be removed entirely.

[1] https://github.com/w3c/csswg-drafts/issues/3932
[2] https://chromium-review.googlesource.com/c/chromium/src/+/2658617/
[3] https://hackmd.io/@dazabani/cr-highlight-pseudos-2021-06#text-shadow-property

Bug: 1164461
Change-Id: I8253ad2f3054320c5aba452296be0c72f0015a6c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3222139
Commit-Queue: Dan Clark <daniec@microsoft.com>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Fernando Fiori <ffiori@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#931761}

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wpt-commits: e6042b2aa44d5ee0a2ebcfffd0612ee933675f89
wpt-pr: 31248