Metal: re-enable retained references.

A client of Flutter is seeing a crash when trying to release the
MTLTexture in GrMtlAttachment. The range of CLs involved includes the
one that disables retained references in the command buffer, so this
is a speculative change to see if that is the culprit.

Change-Id: Idd6e6b264086d671be52f456ef56f1fc5bafade8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/444498
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
(cherry picked from commit 79ff8f20bf26452fd5f0a1357861ebc103a86ab6)
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/446178
Auto-Submit: Jim Van Verth <jvanverth@google.com>