[SRI Message Signature] Cancel requests early when possible.

We send integrity assertions to the network service in order to support
the `Accept-Signature` header. We can use that information to cancel
mismatched responses as soon as headers have been delivered, rather than
waiting until we hit Blink.

We still need to keep checks in Blink to handle cached resources whose
current integrity assertions are distinct from the assertions they were
originally delivered with, but this should speed things up in the general
case.

This is the Chromium side of a two-sided patch:

1.  (Chromium) https://crrev.com/c/6394978 [You are here]
2.  (Devtools) https://crrev.com/c/6396680

Bug: 406424762
Change-Id: I856355edae0b8cef54f9e591546b32d28a7358d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6394978
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: Danil Somsikov <dsv@chromium.org>
Reviewed-by: Joe DeBlasio <jdeblasio@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1438569}