Fetch: Plumb request initiator through passthrough service workers.

This CL contains essentially two changes:

1. The request initiator origin is plumbed through service workers
   that do `fetch(evt.request)`.  In addition to plumbing, this
   requires changes to how we validate navigation requests in the
   CorsURLLoaderFactory.
2. Tracks the original destination of a request passed through a
   service worker.  This is then used in the network service to force
   SameSite=Lax cookies to treat the request as a main frame navigation
   where appropriate.

For more detailed information about these changes please see the
internal design doc at:

https://docs.google.com/document/d/1KZscujuV7bCFEnzJW-0DaCPU-I40RJimQKoCcI0umTQ/edit?usp=sharing

In addition, there is some discussion of these features in the following
spec issues:

https://github.com/whatwg/fetch/issues/1321
https://github.com/whatwg/fetch/issues/1327

The test includes WPT tests that verify navigation headers and SameSite
cookies.  Note, chrome has a couple expected failures in the SameSite
cookie tests because of the "lax-allowing-unsafe" intervention that is
currently enabled.  See:

https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/web_tests/TestExpectations;l=4635;drc=e8133cbf2469adb99c6610483ab78bcfb8cc4c76

Bug: 1115847,1241188
Change-Id: I7e236fa20aeabb705aef40fcf8d5c36da6d2798c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3115917
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Cr-Commit-Position: refs/heads/main@{#936029}