Re: [w3ctag/design-reviews] COEP reflection (Issue #742)

For this use case in particular, I will ask them to reply directly here.

My recollection is that for publishers who want to continue monetizing their site and use [SharedArrayBuffers](https://web.dev/coop-coep/), they have no choice other than registering to the [reverse origin trial](https://developer.chrome.com/origintrials/#/view_trial/303992974847508481). It was meant to be temporary, we need to close it. If no solution is provided, they will be broken. We identified anonymous iframe as a solution.

For ads, knowing if COEP is enabled allows to know beforehand the <iframe> will be blocked by COEP, and use anonymous iframe as an alternative.
If COEP reflection is canceled here, I see those alternatives:
1. Use anonymous iframe unconditionally. This is almost similar to getting a stronger version of [storage partitioning](https://privacycg.github.io/storage-partitioning/) in advance + losing autofill + losing popups with opener. There might be also some performance cost due to additional partitioning. I am not sure they can afford it.
2. Use the polyfill described above. Issues are delay and noises in devtools/reports.
3. Using `window.crossOriginIsolated` as a proxy for `window.crossOriginEmbedderPolicy`. Publisher using COEP without COOP would be left without a solution.
4. Ask individual publisher to expose their COEP state to the ads script.

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