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This makes PrinterProviderAPI a pure interface, with the
implementation hidden in printer_provider.cc, and with a custom browser
context keyed service factory. This will make it easier to inject a
fake API implementation in tests (e.g. for ExtensionPrinterHandler).

BUG=461114
TEST=extensions_browsertests --gtest_filter=PrinterProviderAPI

Review URL: https://codereview.chromium.org/922833004

Cr-Commit-Position: refs/heads/master@{#318124}