bluetooth: Change GetInquiryTxPower and GetInquiryRSSI to return optional.

device/bluetooth/bluetooth_low_energy_device_mac.mm

Index: device/bluetooth/bluetooth_low_energy_device_mac.mm
diff --git a/device/bluetooth/bluetooth_low_energy_device_mac.mm b/device/bluetooth/bluetooth_low_energy_device_mac.mm
index 0db9ce10233571d642f9c29ee3050faecdf8b970..32259983c6542f230183ac0b365172f2fd56a821 100644
--- a/device/bluetooth/bluetooth_low_energy_device_mac.mm
+++ b/device/bluetooth/bluetooth_low_energy_device_mac.mm
@@ -143,13 +143,14 @@ bool BluetoothLowEnergyDeviceMac::IsConnecting() const {
   return ([peripheral_ state] == CBPeripheralStateConnecting);
 }
 
-int16_t BluetoothLowEnergyDeviceMac::GetInquiryRSSI() const {
-  return kUnknownPower;
+base::Optional<int8_t> BluetoothLowEnergyDeviceMac::GetInquiryRSSI() const {
+  NOTIMPLEMENTED();
+  return base::nullopt;
 }
 
-int16_t BluetoothLowEnergyDeviceMac::GetInquiryTxPower() const {
+base::Optional<int8_t> BluetoothLowEnergyDeviceMac::GetInquiryTxPower() const {
   NOTIMPLEMENTED();
-  return kUnknownPower;
+  return base::nullopt;
 }
 
 bool BluetoothLowEnergyDeviceMac::ExpectingPinCode() const {