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This is a reland of 0588d062, as it was concluded
not to have been the culprit for the compile breakage.

Original change's description:
> Introduce a generic ScopedU2fDiscoveryFactory override.
>
>  -- 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}

TBR=jdoerrie@chromium.org

Bug: 785955
Change-Id: I4c9f42d81793567b6536d303f8a4d020fbe147d9
Reviewed-on: https://chromium-review.googlesource.com/959322
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542596}