The WPT test domains don't qualify for site isolation, since they don't differ in more than eTLD+1. Currently, trying to test a web platform test with site isolation results in a command like this:

out/Default/content_shell <your args here...> --isolate-origins=http://www.web-platform.test:8001/,http://www1.web-platform.test:8001/,http://www2.web-platform.test:8001/,http://xn--n8j6ds53lwwkrqhv28a.web-platform.test:8001/,http://xn--lve-6lad.web-platform.test:8001/,http://www.web-platform.test:8081/,http://www1.web-platform.test:8081/,http://www2.web-platform.test:8081/,http://xn--n8j6ds53lwwkrqhv28a.web-platform.test:8081/,http://xn--lve-6lad.web-platform.test:8081/,https://www.web-platform.test:8444/,https://www1.web-platform.test:8444/,https://www2.web-platform.test:8444/,https://xn--n8j6ds53lwwkrqhv28a.web-platform.test:8444/,https://xn--lve-6lad.web-platform.test:8444/

I guess there will be some tests that won't work with everything isolated (i.e. anything that tests document.domain) but maybe we can isolate all the WPT test domains for the other tests.