Re: [whatwg/webidl] Use SafePromiseResolve to replace the promise resolution steps (PR #1645)

noamr left a comment (whatwg/webidl#1645)

I checked the web compat impact of this change against the HTTP Archive crawl with an instrumented Chromium build. I couldn't find any page where it would make an observable difference: in a sample of 5,512 root pages, none resolved a Web IDL promise with a value whose `then` lookup could run script.

AI summary of the research ([noamr/webidl-safe-resolve-compat-1645](https://github.com/noamr/webidl-safe-resolve-compat-1645)):

> **Method.** A temporary probe in Blink's Web IDL promise resolution (`ScriptPromiseResolver`, `ScriptPromise::FromV8Value`) walks each resolution value's prototype chain without running script, and flags a `then` accessor, a Proxy, or a legacy platform object (Blink objects with named or indexed interceptors). It also counts ordinary thenables. A build flag applying the proposal's deferral was checked on a local test page. I loaded a seeded random sample of 6,000 root pages from the October 2026 crawl (300 per rank bucket and client) in that build; 5,512 loaded. Separately, a search of HTTP Archive's ~20k-page sample found 11 pages whose scripts define a `then` getter, and I loaded those too.
>
> | | Pages |
> |---|---|
> | Loaded | 5,512 |
> | Resolved a Web IDL promise with a value whose `then` lookup could run script | 0 (95% upper bound 0.054%, unweighted) |
> | Resolved with an ordinary thenable (`then` still called in the same microtask) | 656 |
>
> The `then` getters found in page scripts (SvelteKit's lazily thenable objects, a video player library) didn't reach Web IDL promise resolution on any of the 11 pages. Pages were loaded without interaction, so resolutions that only happen after user input aren't covered, and there is no use counter for a field rate.
>
> Details: [README](https://github.com/noamr/webidl-safe-resolve-compat-1645/blob/main/README.md)


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