Bug 1627103 [wpt PR 22667] - Reland "[EventTiming] Implement EventCounts", a=testonly

Bug 1627103 [wpt PR 22667] - Reland "[EventTiming] Implement EventCounts", a=testonly

Automatic update from web-platform-tests
Reland "[EventTiming] Implement EventCounts"

This is a reland of 7d25a90d61ddc8c7475d96f84173f925e1eb2d4e

The test needed some fixes.

Original change's description:
> [EventTiming] Implement EventCounts
>
> This CL adds support for performance.eventCounts, gated behind the
> EventTiming flag. EventCounts is maplike and enables computing the
> number of input events that have occurred for any of the supported event
> types. It is populated on WindowPerformance::RegisterEventTiming so that
> it matches when timing with when the PerformanceEventTiming entry is
> created for slow events. The spec for this is here:
> https://wicg.github.io/event-timing/#sec-event-counts
>
> Bug: 543598
>
> Change-Id: I95bd8b977556557811a1eff8d9fc5d106af0d2b1
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2125019
> Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
> Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#755468}

Bug: 543598
Change-Id: I2dd1ea4379805c246976746ffb6d02346adb1141
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2134931
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757102}

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wpt-commits: 8687eeb366f4e35ce34e07607f9850211e3d6dd8
wpt-pr: 22667