For some reason on macOS with ASAN enabled, the test promises in
HTML/Window-postMessage do not resolve. These promises wait for both
the in-document and the blob url iframes to load. It's not clear why
this works fine in Linux nor why the onload handler doesn't fire
when the iframe has already loaded.
This aligns Workers and Window and MessagePorts to all use the same
mechanism for transferring serialized messages across realms.
It also allows transferring more message ports into a worker.
Re-enable the Worker-echo test, as none of the MessagePort tests have
themselves been flaky, and those are now using the same underlying
implementation.
