bluetooth: Return int8_t and use -128 for unknown tx power.

device/bluetooth/bluetooth_low_energy_device_mac.mm

 // 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.
 
 #include "device/bluetooth/bluetooth_low_energy_device_mac.h"
 
 #import <CoreFoundation/CoreFoundation.h>
 #include <stddef.h>
 
 #include "base/mac/mac_util.h"
@@ skipping 127 matching lines @@
 
 bool BluetoothLowEnergyDeviceMac::IsConnecting() const {
   return ([peripheral_ state] == CBPeripheralStateConnecting);
 }
 
 BluetoothDevice::UUIDList BluetoothLowEnergyDeviceMac::GetUUIDs() const {
   return BluetoothDevice::UUIDList(advertised_uuids_.begin(),
                                    advertised_uuids_.end());
 }
 
-int16_t BluetoothLowEnergyDeviceMac::GetInquiryRSSI() const {
-  return kUnknownPower;
+base::Optional<int8_t> BluetoothLowEnergyDeviceMac::GetInquiryRSSI() const {
+  return base::Optional<int8_t>();
 }
 
-int16_t BluetoothLowEnergyDeviceMac::GetInquiryTxPower() const {
+base::Optional<int8_t> BluetoothLowEnergyDeviceMac::GetInquiryTxPower() const {
   NOTIMPLEMENTED();
-  return kUnknownPower;
+  return base::Optional<int8_t>();
 }
 
 bool BluetoothLowEnergyDeviceMac::ExpectingPinCode() const {
   return false;
 }
 
 bool BluetoothLowEnergyDeviceMac::ExpectingPasskey() const {
   return false;
 }
 
@@ skipping 106 matching lines @@
 
 void BluetoothLowEnergyDeviceMac::DidDisconnectPeripheral(
     BluetoothDevice::ConnectErrorCode error_code) {
   if (create_gatt_connection_error_callbacks_.empty()) {
     // TODO(http://crbug.com/585897): Need to pass the error.
     DidDisconnectGatt();
   } else {
     DidFailToConnectGatt(error_code);
   }
 }