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The [coop-reporting-explainer] defined several URLs that might be
reported:
- initial_popup_url
- openee_url
- opener_url
- other_document_url
- referrer

This patch adds body.initialPopupURL

[coop-reporting-explainer]:
https://github.com/camillelamy/explainers/blob/master/coop_reporting.md

Bug: 1090273
Change-Id: I535e44e66578fc90f449be59586f19b154677caf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424092
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812588}