Replace protobuf's StatusOr stub with base::expected

Protobuf's stubs do not appear to be public API, as they've changed in
the newest release. As I understand, they're just temporary standins
until protobuf takes a real dep on Abseil, etc. Which means this code
should just be using real absl::StatusOr.

However, Chromium hasn't allowed absl::StatusOr yet, and [1] suggests
using base::expected. Indeed, this code doesn't actually use
absl::Status meaningfully. It only uses it as a way to store a
std::string error, so replace it with expected<T, string>.

[1] https://groups.google.com/a/chromium.org/g/cxx/c/ucsnOkZhS2c/m/GN7suzDiAQAJ

Bug: 1294200
Change-Id: Ie6d45ff882b32b7bf6f73f3c9fa889f6ffca55e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3696716
Commit-Queue: David Benjamin <davidben@chromium.org>
Reviewed-by: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1013980}