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.
The main issues are using Structured{Serialize,Deserailize} instead of
Structured{Serialize,Deserialize}WithTransfer and the temporary
execution context usage for StructuredDeserialize.
Allows Discord to load once again, as it uses a postMessage scheduler
to render components, including the main App component. The callback
checked the (previously) non-existent source attribute of the
MessageEvent and returned if it was not the main window.
Fixes the Twitch cookie consent banner saying "failed integrity check"
for unknown reasons, but presumably related to the source and origin
attributes.
