Revert "Refactor Suggestion::Payload to use StrongAlias for BackendId instead of a std::string"

This reverts commit a47fddbf85515335662a28b9a4268ffd181aeefd.

Reason for revert: breaks debug Mac build: https://ci.chromium.org/ui/p/chromium/builders/ci/GPU%20FYI%20Mac%20Builder%20(dbg)/171791/blamelist

Original change's description:
> Refactor Suggestion::Payload to use StrongAlias for BackendId instead of a std::string
>
> This CL refactors the payload of a suggestion to use a StrongAlias for BackendId instead of an std::string, so that it is more descriptive.
>
> This change should be a no-op.
>
> Fixed: 1349133
> Change-Id: I8bbc84fa2f2a82d705088f0ffc0077782e83c026
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3715236
> Reviewed-by: Christoph Schwering <schwering@google.com>
> Commit-Queue: Vinny Persky <vinnypersky@google.com>
> Reviewed-by: Siyu An <siyua@chromium.org>
> Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1036318}

Change-Id: I4183ad4d4d1162ffbc67ad35cd9335f5a65993fa
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3834386
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Owners-Override: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1036333}