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Previously, we were constructing and returning a unique_ptr solely for
the purpose of being able to return a nullptr if a printer was not found.
Instead, let's just return an optional which makes more sense for this.

Change-Id: I14db20b6f7ffc8855ee48de2eda622fb78ec6b57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1572263
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652894}