webusb: Reject the promise if the device is already open.

device/usb/public/interfaces/device.mojom

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module device.usb;
 
 enum OpenDeviceError {
   // Opening the device succeeded.
   OK,
 
   // The operating system denied access to the device.
   ACCESS_DENIED,
+
+  // The device is already open.
+  ALREADY_OPEN,
 };
 
 enum TransferDirection {
   INBOUND,
   OUTBOUND,
 };
 
 enum ControlTransferType {
   STANDARD,
   CLASS,
@@ skipping 253 matching lines @@
   // indicates no timeout: the request will remain pending indefinitely until
   // completed or otherwise terminated.
 
   // |packets| contains the status of each packet sent to the device, in order.
   IsochronousTransferOut(uint8 endpoint_number,
                          array<uint8> data,
                          array<uint32> packet_lengths,
                          uint32 timeout)
       => (array<IsochronousPacket> packets);
 };