Have chrome_desktop_report_request_helper.cc use SecKeychain directly

chrome_desktop_report_request_helper.cc relies on `AppleKeychain` being
implemented in terms of the deprecated `SecKeychain` APIs and providing
access to the underlying `SecKeychainItemRef` to use it to make direct
`SecKeychain` calls.

`AppleKeychain` will soon switch to wrapping the `SecItem` APIs which do
not support returning `SecKeychainItemRef` objects.

chrome_desktop_report_request_helper.cc will need to be updated to use
`SecItem` APIs independently. This work is tracked by b/405511914. Until
then, having it use `SecKeychain` directly unblocks migrating
`AppleKeychain` to `SecItem`.

Once everything is updated to `SecItem` it may be possible to re-unify
its use with `AppleKeychain`.

Bug: 395659797, b/405511914
Change-Id: I6a6a2100871353b4f21b108f75e5f89f329eedef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6443471
Reviewed-by: Dominique Fauteux-Chapleau <domfc@chromium.org>
Commit-Queue: Mark Rowe <markrowe@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1445271}