Revert "Temporarily revert runtime decoder registration."

This reverts commit bcbf9633db44a89aebc9b56a80354d652401b01b.

Next patch set implements a local shared spinlock
rather than depending on SkSharedMutex.

Next patch removes the thread safety and the unit test.

Original change's description:
> Temporarily revert runtime decoder registration.
>
> Some builds in Google3 can't find SkSharedMutex at link time.
>
> This reverts commit ff7006ff8082a6f4298cc9f32acc726535d93711.
> This reverts commit 05f45b21b72fdf738986ea428346f2fbec8444d8.
>
> Change-Id: I889e9a3169cf017b8d5daacf75099663fc996d73
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/210785
> Reviewed-by: Mike Klein <mtklein@google.com>
> Commit-Queue: Mike Klein <mtklein@google.com>

Change-Id: I6df8e2dfb64a92c780b62acddf7a797f24ba8e31
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/210792
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed