Remove window name on cross origin navigation: fix DCHECK crash

fix failed dcheck in https://source.chromium.org/chromium/chromium/src/+/refs/tags/140.0.7339.81:third_party/blink/renderer/core/frame/web_local_frame_impl.cc;l=2170
and adds a way to stay informed about the activation of MPArch (https://docs.google.com/document/d/1kAMkME-WGLX33ai7p6VBhUXM_iCBjBzRjHH-OynwBNA)