CrosNetworkConfig: Return early after failing to GetManagedProperties.

CrosNetworkConfig::OnGetManagedProperties was running OnceCallback
twice under certain circumstances (like forgetting a disconnected VPN
network), causing chrome to crash. Added a return statement after the
first time the OnceCallback is run.

Bug: 1111431
Change-Id: Ib901bed15c2ec0923915f0963cc27563fb3162e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2329924
Commit-Queue: Claude van der Merwe <cvandermerwe@google.com>
Reviewed-by: Jon Mann <jonmann@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794640}

chromeos/services/network_config/cros_network_config.cc

@@ -1888,6 +1888,7 @@ void CrosNetworkConfig::OnGetManagedProperties(
     NET_LOG(ERROR) << "GetManagedProperties failed for: " << guid
                    << " Error: " << error.value_or("Failed");
     std::move(callback).Run(nullptr);
+    return;
   }
   const NetworkState* network_state =
       network_state_handler_->GetNetworkState(service_path);