[M96] Deleting unused field: FetchEventPreloadHandle::url_loader.

The FetchEventPreloadHandle::url_loader field is not really used - it
is only needed to keep the URLLoader alive (and this can be accomplished
in a simpler way, by keeping the mojo::PendingRemote in the Browser
process).

This CL removes the FetchEventPreloadHandle::url_loader field and the
FetchEventPreloadHandle and WebFetchEventPreloadHandle types (collapsing
these handle types into their only other remaining field:
mojo::PendingReceivernetwork::mojom::URLLoaderClient).

(cherry picked from commit dbe67ccde52e30acf6a66b1b9cc83768a067fa6a)

Fixed: 1264477
Change-Id: I9c9c54900d79e92ac08eeb43536c938fa84a58f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3252914
Reviewed-by: Daniel Cheng dcheng@chromium.org
Reviewed-by: Ben Kelly wanderview@chromium.org
Reviewed-by: Hiroki Nakagawa nhiroki@chromium.org
Commit-Queue: Łukasz Anforowicz lukasza@chromium.org
Cr-Original-Commit-Position: refs/heads/main@{#937895}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3262600
Bot-Commit: Rubber Stamper rubber-stamper@appspot.gserviceaccount.com
Commit-Queue: Ben Kelly wanderview@chromium.org
Cr-Commit-Position: refs/branch-heads/4664@{#853}
Cr-Branched-From: 24dc4ee75e01a29d390d43c9c264372a169273a7-refs/heads/main@{#929512}

Notes: Backported fix for CVE-2021-38010.