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It was found that BulkPrintersCalculatorFactory is leaking state between
tests and causing flakiness.  Change ownership from a singleton to an
object owned by chrome_browser_main_parts_chromeos to guarantee that only
one ever exists during tests and in production.  To accommodate this, adjust
users of the factory to handle a null factory which will occur during unit
tests and shutdown.

Bug: chromium:1015949, chromium:966561
Change-Id: I61eb5bc9e37363e95d26e8b07ae0cb0d5087c275
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1888517
Commit-Queue: Sean Kau <skau@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Nikita Podguzov <nikitapodguzov@chromium.org>
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721562}