Sync web-platform-tests PR 21640 into mozilla-central (this bug is closed when the sync is complete).

PR: https://github.com/web-platform-tests/wpt/pull/21640
Details from upstream follow.

Robert Ma <robertma@chromium.org> wrote:

Revert "Move SMIL events tests to WPT"

This reverts commit 6a196c8134820a3c7f81d99857117447b4e80925.

Reason for revert: Unfortunately SVG files are not recognized as
tests in WPT, so moving these files into WPT actually means that we
stop running them altogether. See this CQ run:
https://ci.chromium.org/p/chromium/builders/try/linux-rel/303153

Original change's description:

Move SMIL events tests to WPT

Change-Id: Ie4c00b4b6b6eba3b55f86793620a502780590864
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2041270
Reviewed-by: Stephen Chenney \<schenney@chromium.org>
Commit-Queue: Fredrik Söderquist \<fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#738955}

TBR=pdr@chromium.org,fs@opera.com,schenney@chromium.org

Change-Id: I09a5cde1c656ba765b710b7d5f01dd588c13da51
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2042125
Reviewed-by: Robert Ma \<robertma@chromium.org>
Commit-Queue: Robert Ma \<robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739048}

https://hg.mozilla.org/mozilla-central/rev/27098a0646f1