Serve benchmarks with COOP/COEP headers in wasm mode.

[eyebrowsoffire]

We need crossOriginIsolated for skwasm to work. Otherwise, this just falls back and benchmarks JS + canvaskit.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing. The test exemption team is a small volunteer group, so all reviewers should feel empowered to ask for tests, without delegating that responsibility entirely to the test exemption group.

[yjbanov]

Can we test this by doing a simple dart:io HTTP fetch using a Dart test and checking for headers?

[eyebrowsoffire]

Can we test this by doing a simple dart:io HTTP fetch using a Dart test and checking for headers?

I think I'm actually going to add some data to the benchmark payload that says some info about the environment, including whether it is wasm. Then I'll go ahead and confirm it matches that in the tests. I think that's a bit more comprehensive and gives me a better level of confidence than just inspecting the HTTP headers.