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 -- In preparation for introducing ScopedU2fVirtualDiscoveryFactory, create a
    base class ScopedU2fDiscoveryFactory and let ScopedU2fFakeDiscoveryFactory
    derive from that.
 -- In preparation for a wider adoption of FakeU2fDiscovery in tests that could
    not care less about when a discovery starts, add a version of the fake
    discovery where SimulateStarted()/SimulateStopped() is called automatically.

Bug: 785955
Change-Id: I4b1c9bd962c6e225be45971bbb0d5aad3b05a68a
Reviewed-on: https://chromium-review.googlesource.com/959001
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542507}