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This adds a FidoDiscovery::Stop() method that can be used to terminate
authenticator discovery without destroying the instance.

FidoDeviceDiscovery upon receiving Stop() will ignore all calls to
AddDevice() and RemoveDevice(). FidoCableDiscovery inherits this
behavior and also calls Unregister() on all pending BleAdvertisements.

Lastly, AuthenticatorCommon is changed to stop all discoveries of the
current FidoRequestHandler prior to signaling failure of the request to
the UI layer.

This fixes an issue where Chrome might continue to advertise caBLE EIDs
while the UI is showing an error sheet, e.g. because the request timed
out or because one of the authenticator returned an error that causes
the UI to show its retry error sheet.

Bug: 1046164
Change-Id: Ic5678f13a10efbb56277e6bd3485dd985c8ab0c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036266
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739073}