Fix output buffering for cross-origin isolation

[swissspidy]

What?

This is a quick follow-up to #64650 with two simple fixes:

1. Ensures the HTTP headers for cross-origin isolation are correctly sent in the output buffer callback
2. Adds some hardening to ensure wasm files are served with the correct mime type (if not already done by the host anyway)
   • Actual wasm files will be added in follow-up PRs.

Note: this is only applicable when the client-side media processing experiment is enabled.

Why?

See #61447

How?

Testing Instructions

N/A

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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Co-authored-by: swissspidy <swissspidy@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>
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[felixarntz]

Two questions, but overall LGTM.

[felixarntz]

Can you explain why this is no longer relevant?

I wouldn't have asked if it hadn't been here before, but why was it there before and is now being removed?

[swissspidy]

Well it was there before because I thought I needed it, but turns out it doesn't work, so I just removed it.

[westonruter]

FWIW, the PL plugin includes this: WordPress/performance#1443

And the Optimization Detective plugin: WordPress/performance#1317

In my testing it worked, although I wrestled with this one for awhile to replicate the scenario where it would be relevant.

[swissspidy]

IDK what's different here, but this change here fixes the issue for me and would unblock further work, so I'd appreciate an approval