[CSP] Fix flaky WPTs for history inheritance

The tests were flaky because we were awaiting messages from another
window and checking msg.source while the window was being navigated.
This CL fixes that by instead attaching a token to every involved
window, and checking the token for attributing messages to the right
sender.

Fixed: 1274917
Change-Id: I3d890feebcc4fbdcf390693a2838ce9c03f831fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3497689
Commit-Queue: Antonio Sartori <antoniosartori@chromium.org>
Reviewed-by: Titouan Rigoudy <titouan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#977197}