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This moves the static factory methods for instantiating subclasses of
FidoDeviceDiscovery out of FidoDeviceDiscovery and into a new class called
FidoDiscoveryFactory. The ScopedFidoDeviceDiscoveryFactory is moved along with
it.

This will simplify changes that let us instantiate non-device authenticators
(Windows, Touch ID) through the existing factory methods. Also, instantiating
different kinds of FidoDiscovery is becoming complex enough that centralizing
this code in a single place, separate from any of the FidoDiscovery
implementations seems sensible.

Bug: 898718
Change-Id: I6c114c5aa9e8db4538a003108c4e7acec7100fbd
Reviewed-on: https://chromium-review.googlesource.com/c/1311841
Commit-Queue: Martin Kreichgauer <martinkr@chromium.org>
Reviewed-by: Kim Paulhamus <kpaulhamus@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604806}