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

device/bluetooth/test/mock_bluetooth_device.h

Index: device/bluetooth/test/mock_bluetooth_device.h
diff --git a/device/bluetooth/test/mock_bluetooth_device.h b/device/bluetooth/test/mock_bluetooth_device.h
index f6e79ec5309353508ce49753971546b59a9ab62e..ae5a9e09f13d5a6ba6f6477057653547941a273e 100644
--- a/device/bluetooth/test/mock_bluetooth_device.h
+++ b/device/bluetooth/test/mock_bluetooth_device.h
@@ -49,8 +49,8 @@ class MockBluetoothDevice : public BluetoothDevice {
   MOCK_CONST_METHOD0(IsConnectable, bool());
   MOCK_CONST_METHOD0(IsConnecting, bool());
   MOCK_CONST_METHOD0(GetUUIDs, UUIDList());
-  MOCK_CONST_METHOD0(GetInquiryRSSI, int16_t());
-  MOCK_CONST_METHOD0(GetInquiryTxPower, int16_t());
+  MOCK_CONST_METHOD0(GetInquiryRSSI, base::Optional<int8_t>());
+  MOCK_CONST_METHOD0(GetInquiryTxPower, base::Optional<int8_t>());
   MOCK_CONST_METHOD0(ExpectingPinCode, bool());
   MOCK_CONST_METHOD0(ExpectingPasskey, bool());
   MOCK_CONST_METHOD0(ExpectingConfirmation, bool());