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This is a single browser test that verifies CLS metric values through
four reporting mechanisms (web perf API, trace event, UKM, and UMA).

Bug: 1045064
Change-Id: Ie1616e961b2771e792d9d2b15babaaa2125822d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1943427Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739035}