Change base::expected to use std::variant under the hood

It previously used absl::variant, which appears not to be as constexpr
friendly. E.g., it is possible for a constexpr function to return
base::expected<void, std::string> when using std::variant, but not when
using absl::variant.

Additionally adds the include for absl::variant to a few files that were
previously relying on it being included transitively via
base/types/expected.h so they continue to compile, and removes one
instance of an unused variable that the compiler can properly detect now
that std::variant is used.

Change-Id: Ibab13382202c69fd45c5aff318e8f56b3ac0d30f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6345816
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Owners-Override: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1431735}