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

device/bluetooth/bluetooth_low_energy_device_mac.h

 // 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.
 
 #ifndef DEVICE_BLUETOOTH_BLUETOOTH_LOW_ENERGY_DEVICE_MAC_H_
 #define DEVICE_BLUETOOTH_BLUETOOTH_LOW_ENERGY_DEVICE_MAC_H_
 
 #if defined(OS_IOS)
 #import <CoreBluetooth/CoreBluetooth.h>
 #else  // !defined(OS_IOS)
@@ skipping 35 matching lines @@
   uint16_t GetVendorID() const override;
   uint16_t GetProductID() const override;
   uint16_t GetDeviceID() const override;
   uint16_t GetAppearance() const override;
   bool IsPaired() const override;
   bool IsConnected() const override;
   bool IsGattConnected() const override;
   bool IsConnectable() const override;
   bool IsConnecting() const override;
   BluetoothDevice::UUIDList GetUUIDs() const override;
-  int16_t GetInquiryRSSI() const override;
-  int16_t GetInquiryTxPower() const override;
+  base::Optional<int8_t> GetInquiryRSSI() const override;
+  base::Optional<int8_t> GetInquiryTxPower() const override;
   bool ExpectingPinCode() const override;
   bool ExpectingPasskey() const override;
   bool ExpectingConfirmation() const override;
   void GetConnectionInfo(const ConnectionInfoCallback& callback) override;
   void Connect(PairingDelegate* pairing_delegate,
                const base::Closure& callback,
                const ConnectErrorCallback& error_callback) override;
   void SetPinCode(const std::string& pincode) override;
   void SetPasskey(uint32_t passkey) override;
   void ConfirmPairing() override;
@@ skipping 67 matching lines @@
 
   // The services (identified by UUIDs) that this device provides.
   std::set<BluetoothUUID> advertised_uuids_;
 
   DISALLOW_COPY_AND_ASSIGN(BluetoothLowEnergyDeviceMac);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_BLUETOOTH_BLUETOOTH_LOW_ENERGY_DEVICE_MAC_H_