Fixing a WPT timeout in Window.postMessage()

Let's fix a failing test on WPT to make number go up! PR: https://github.com/LadybirdBrowser/ladybird/pull/1615 WPT dashboard: https://wpt.fyi/results/?product=ladybird Ladybird: https://ladybird.org/ GitHub: https://github.com/LadybirdBrowser/ladybird/ Discord: https://discord.gg/nvfjVJ4Svh I'm using the CLion IDE from JetBrains: https://jb.gg/clion_ak My coding font is Berkeley Mono: https://berkeleygraphics.com/typefaces/berkeley-mono/