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

device/bluetooth/bluetooth_device_win.cc

Index: device/bluetooth/bluetooth_device_win.cc
diff --git a/device/bluetooth/bluetooth_device_win.cc b/device/bluetooth/bluetooth_device_win.cc
index 525ab0a1d9da8f5e2aa24adcabc9d13e46845e1d..f7105989c5631be496472ea8cab7f4662984a4bb 100644
--- a/device/bluetooth/bluetooth_device_win.cc
+++ b/device/bluetooth/bluetooth_device_win.cc
@@ -112,13 +112,13 @@ BluetoothDevice::UUIDList BluetoothDeviceWin::GetUUIDs() const {
   return uuids_;
 }
 
-int16_t BluetoothDeviceWin::GetInquiryRSSI() const {
-  return kUnknownPower;
+base::Optional<int8_t> BluetoothDeviceWin::GetInquiryRSSI() const {
+  return base::Optional<int8_t>();
 }
 
-int16_t BluetoothDeviceWin::GetInquiryTxPower() const {
+base::Optional<int8_t> BluetoothDeviceWin::GetInquiryTxPower() const {
   NOTIMPLEMENTED();
-  return kUnknownPower;
+  return base::Optional<int8_t>();
 }
 
 bool BluetoothDeviceWin::ExpectingPinCode() const {