device/bluetooth: Add abstract API definition for BluetoothGattDescriptor.

device/bluetooth/bluetooth_gatt_service.h

Index: device/bluetooth/bluetooth_gatt_service.h
diff --git a/device/bluetooth/bluetooth_gatt_service.h b/device/bluetooth/bluetooth_gatt_service.h
index ca0f981c1d57a867f86609c3002000f4a4facb27..963fdd50141daa332e525f8595bf65d70c2dd6ff 100644
--- a/device/bluetooth/bluetooth_gatt_service.h
+++ b/device/bluetooth/bluetooth_gatt_service.h
@@ -147,7 +147,7 @@ class BluetoothGattService {
   };
 
   // The ErrorCallback is used by methods to asynchronously report errors.
-  typedef base::Callback<const std::string&> ErrorCallback;
+  typedef base::Callback<void(const std::string&)> ErrorCallback;
 
   // Constructs a BluetoothGattService that can be locally hosted when the local
   // adapter is in the peripheral role. The resulting object can then be made