bluetooth: Remove queuing of GetCharacteristicsCallbacks

device/bluetooth/public/interfaces/device.mojom

 // Copyright 2016 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 bluetooth.mojom;
 
 import "device/bluetooth/public/interfaces/uuid.mojom";
 
 // Values representing the possible properties of a characteristic, which
 // define how the characteristic can be used. Each of these properties serve
@@ skipping 52 matching lines @@
   Disconnect();
 
   // Gets basic information about the device. Returns null, if no device is
   // available.
   GetInfo() => (DeviceInfo? info);
 
   // Gets the GATT Services in this device's GATT Server.
   GetServices() => (array<ServiceInfo> services);
 
   // Gets the GATT Characteristics in the GATT Service with |service_id|.
+  // If |characteristics| is null, an error occured while attempting to retrieve
+  // the array of characteristics. If |characteristics| is empty, this simply
+  // means that no characteristics were found.
   GetCharacteristics(string service_id) =>
-      (array<CharacteristicInfo> characteristics);
+      (array<CharacteristicInfo>? characteristics);
 };