fido: mark transport unavailable if a FidoDiscovery fails to start

A FidoRequestHandler currently reports a FidoTransportProtocol to its
Observer (i.e. the UI layer) as available, even if the FidoDiscovery for
that transport invoked DiscoveryStated() with success=false. It seems
logical in this case to act as if the discovery never had been
instantiated in the first place.

Change-Id: I50c2832f3b5bc321f2b9454e90770204c229b311
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087955
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748874}

device/fido/fido_request_handler_base.cc

@@ -279,8 +279,13 @@ void FidoRequestHandlerBase::DiscoveryStarted(
     FidoDiscoveryBase* discovery,
     bool success,
     std::vector<FidoAuthenticator*> authenticators) {
-  for (auto* authenticator : authenticators) {
-    AuthenticatorAdded(discovery, authenticator);
+  if (!success) {
+    transport_availability_info_.available_transports.erase(
+        discovery->transport());
+  } else {
+    for (auto* authenticator : authenticators) {
+      AuthenticatorAdded(discovery, authenticator);
+    }
   }
   DCHECK(notify_observer_callback_);
   notify_observer_callback_.Run();