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This adds to new events to the WebAuthentication.CableV1DiscoveryEvent
UMA histogram.

kStartScanningFailed (12) indicates that
BluetoothAdapter::StartDiscoverySession() was called but failed.
kScanningStoppedUnexpectedly (13) indicates that after
StartDiscoverySession() succeeded, the BluetoothAdapter notified the
FidoCableDiscovery that the discovery session stopped due to some
error.

Both of those scenarios currently cannot be recovered from without the
user manually retrying the entire request.

Change-Id: Idfd4b9c2f45cb2eb9549caccdeea6c0463e77a5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2258468Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#781428}