chrome.bluetoothLowEnergy: Implement the onDescriptorValueChanged event.

chrome/common/extensions/api/bluetooth_low_energy.idl

 // Copyright 2014 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.
 
 // The <code>chrome.bluetoothLowEnergy</code> API is used to communicate with
-// Bluetooth Smart (Low Energy) devices.
+// Bluetooth Smart (Low Energy) devices using the
+// <a href="https://developer.bluetooth.org/TechnologyOverview/Pages/GATT.aspx">
+// Generic Attribute Profile (GATT)</a>.
 namespace bluetoothLowEnergy {
   // Values representing the possible properties of a characteristic.
   enum CharacteristicProperty {broadcast, read, writeWithoutResponse, write,
                                notify, indicate, authenticatedSignedWrites,
                                extendedProperties, reliableWrite,
                                writeableAuxiliaries};
 
   // Represents a peripheral's Bluetooth GATT Service, a collection of
   // characteristics and relationships to other services that encapsulate
   // the behavior of part of a device.
@@ skipping 200 matching lines @@
     // Fired when a GATT service that was previously discovered on a remote
     // device has been removed.
     // |service| : The GATT service that was removed.
     static void onServiceRemoved(Service service);
 
     // Fired when the value of a remote GATT characteristic changes, either as
     // a result of a read or write request, or a value change notification or
     // indication.
     // |characteristic| : The GATT characteristic whose value has changed.
     static void onCharacteristicValueChanged(Characteristic characteristic);
+
+    // Fired when the value of a remote GATT characteristic descriptor changes,
+    // usually as a result of a read or write request.
+    // |descriptor| : The GATT characteristic descriptor whose value has
+    // changed.
+    static void onDescriptorValueChanged(Descriptor descriptor);
   };
 };