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This change extracts an abstract base class called FidoDiscoveryBase
from FidoDiscovery. The FidoDiscovery::Observer interface is moved into
the base class, and its signature is changed to allow observing the
instantiation and destruction of FidoAuthenticator instances rather than
FidoDevice. I.e. FidoDiscovery::Observer::Device{Added,Removed} becomes
FidoDiscoveryBase::Observer::Authenticator{Added,Removed}.

FidoDiscovery takes on the responsibility of instantiating and owning
FidoAuthenticator instances (more specifically, FidoDeviceAuthenticator
instances). The AuthenticatorMap in FidoRequestHandlerBase now only
holds plain FidoAuthenticator pointers, rather than owning the
unique_ptrs.

This gets us closer to getting platform-y authenticators (Touch ID,
Windows) injected via Discovery implementations, rather than
implementing custom interaction between FidoRequestHandlerBase and
AuthenticatorImpl for them (see
SetPlatformAuthenticatorOrMarkUnavailable).

Better naming might have been "FidoDeviceDiscovery" for FidoDiscovery
and perhaps "FidoDiscovery" for FidoDiscoveryBase, but that would be a
rather large diff.

A sensible additional refactoring might be to make
FidoDeviceAuthenticator own its associated FidoDevice.

Change-Id: I05d9c24819745c784fef3898bec8d6bb63d2f503
Reviewed-on: https://chromium-review.googlesource.com/c/1256016
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Jun Choi <hongjunchoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596882}