Reland "Cross-origin support for window.fence.reportEvent()."

This is a reland of commit 280fe80c.

There is a known flakiness issue with tests that involve loading
cross-origin subframes with remote context executors. The way we
historically have fixed that is by splitting up each promise_test into a
separate file. This CL attempts to fix that flakiness by doing just
that.

Original change's description:
> Cross-origin support for window.fence.reportEvent().
>
> Third-party ad serving (3PAS) lets advertisers manage ads across
> different bidding platforms. However, the event-level reporting API
> (reportEvent()) currently prevents direct reporting from documents
> that are cross-origin to the root ad frame's fenced frame config's
> mapped URL, limiting this functionality for the 3PAS use case. This CL
> extends the reportEvent() API to support this use case.
>
> To allow cross-origin reporting while still preserving privacy, both
> the ad frame and the 3PAS subframe must explicitly opt in. The ad
> frame opts in using a new `Allow-Cross-Origin-Event-Reporting`
> response header, while the 3PAS subframe opts in by calling
> reportEvent() with `crossOriginExposed=true`. If both opt-ins are
> present, then the reportEvent() call from the 3PAS document will
> succeed.
>
> This CL adds histograms to track how many beacons are sent from
> documents that are same-origin and cross-origin to the fenced frame
> config's mapped url.
>
> This CL also expands the fenced frame WPT system to support
> content-initiated navigations of fenced frame contexts to cross-origin
> URLs.
>
> Change-Id: I4e895bec4cb77604e2017cfc67b496d59863a30b
> Bug: 331844131

Bug: 331844131
Change-Id: Iacf17786ba5471faff15252c87e1924698ab4583
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5425335


Reviewed-by: Dominic Farolino <dom@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Garrett Tanzer <gtanzer@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Liam Brady <lbrady@google.com>
Cr-Commit-Position: refs/heads/main@{#1283165}