Right now Window::post_message passes None as the value that will initialize MessageEvent.origin. The spec says this should be the origin of the source document that posted the message instead. We will want to add a new origin argument to Window::post_message, and store the origin in ScriptMsg::PostMessage as well so the code path in DissimilarOriginWindow::post_message also works.

This should allow tests/wpt/web-platform-tests/dom/events/EventListener-incumbent-global-1.sub.html to make more progress.

Code: components/script/dom/window.rs, components/script/dom/dissimilaroriginwindow.rs, components/script/script_thread.rs, components/script_traits/lib.rs
Test: ./mach test-wpt tests/wpt/web-platform-tests/dom/events/EventListener-incumbent-global-1.sub.html