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- Previous implementation of detected_printer_ppd_references_ used UniquePtrs
  to simulate optional PpdReference values to a map.
- This change changes the UniquePtr to base::Optional.

Bug: 932202
Test: browser_tests --gtest_filter=CrSettingsPrinting*
Change-Id: I9131bef5140ea0c3cb115c489975fae959bea21e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1560494
Commit-Queue: jimmy gong <jimmyxgong@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650527}