Use Uint8Array.fromBase64 instead of atob

[Constellation]

ECMAScript now has base64 -> Uint8Array conversion function in the standard. Let's just use it instead of relying on DOM's atob.

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[danleh]

As of 2025-07-01 Uint8Array.fromBase64 is still behind a flag in V8 (--js-base-64, see this test or the flag definitions here).

It is currently staged, and expected to become available quite soon, see https://crbug.com/42204568:

we will turn the flag on by default sometime next week on Chrome M140 . That means it will be available in Canary very soon, on Aug 6th in Beta and on Aug 27th in Stable.

So the failing v8 build should be fixed around that time, then we can merge it from my side. (Not sure about SpiderMonkey, how's availability there?)

[danleh]

This should be working now in d8 since the --js-base-64 flag is now on-by-default. Let's rerun checks and then merge.

[danleh]

Ah, too early still. While it landed in d8, not yet in Chrome...

[anba]

(Not sure about SpiderMonkey, how's availability there?)

Shipped in Firefox 133. https://bugzilla.mozilla.org/show_bug.cgi?id=1917885

[camillobruni]

Looks like indeed this is no longer called / used: https://github.com/search?q=repo%3AWebKit%2FJetStream%20base64ToArrayBuffer&type=code

I guess we could just remove it then?

[danleh]

The function base64ToArrayBuffer is being used in the tfjs Wasm workloads, but the JS files are so large that they are not indexed by GitHub, see https://github.com/WebKit/JetStream/blob/f75f42a79e3a8f9f903805a61afa18c7045b984a/wasm/tfjs-model-use.js
However, the workload is already disabled by-default, we might as well remove the workload completely.

[danleh]

Let's do that in an (optional) follow-up and for now just merge this PR. Let me close and re-open to trigger CI/actions and see if it now works in Chrome (it should, as M140 is already stable since a while).