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

device/bluetooth/bluetooth_device.h

Index: device/bluetooth/bluetooth_device.h
diff --git a/device/bluetooth/bluetooth_device.h b/device/bluetooth/bluetooth_device.h
index 2d7f4b96d6991375155559beb44fb2ee72ea7b53..92513ce6eee42a73a176ab52c8a1650059735427 100644
--- a/device/bluetooth/bluetooth_device.h
+++ b/device/bluetooth/bluetooth_device.h
@@ -17,6 +17,7 @@
 #include "base/containers/scoped_ptr_hash_map.h"
 #include "base/gtest_prod_util.h"
 #include "base/memory/ref_counted.h"
+#include "base/optional.h"
 #include "base/strings/string16.h"
 #include "device/bluetooth/bluetooth_export.h"
 #include "device/bluetooth/bluetooth_uuid.h"
@@ -77,18 +78,18 @@ class DEVICE_BLUETOOTH_EXPORT BluetoothDevice {
     DEVICE_KEYBOARD_MOUSE_COMBO
   };
 
-  // The value returned if the RSSI or transmit power cannot be read.
-  static const int kUnknownPower = 127;
   // The value returned if the appearance is not present.
   static const uint16_t kAppearanceNotPresent = 0xffc0;
 
   struct DEVICE_BLUETOOTH_EXPORT ConnectionInfo {
-    int rssi;
-    int transmit_power;
-    int max_transmit_power;
+    base::Optional<int8_t> rssi;
+    base::Optional<int8_t> transmit_power;
+    base::Optional<int8_t> max_transmit_power;
 
     ConnectionInfo();
-    ConnectionInfo(int rssi, int transmit_power, int max_transmit_power);
+    ConnectionInfo(const base::Optional<int8_t>& rssi,
+                   const base::Optional<int8_t>& transmit_power,
+                   const base::Optional<int8_t>& max_transmit_power);
     ~ConnectionInfo();
   };
 
@@ -283,15 +284,15 @@ class DEVICE_BLUETOOTH_EXPORT BluetoothDevice {
   virtual UUIDList GetUUIDs() const = 0;
 
   // The received signal strength, in dBm. This field is avaliable and valid
-  // only during discovery. If not during discovery, or RSSI wasn't reported,
-  // this method will return |kUnknownPower|.
-  virtual int16_t GetInquiryRSSI() const = 0;
+  // only during discovery. If not during discovery, or RSSI wasn't reported
+  // this value will be absent.
+  virtual base::Optional<int8_t> GetInquiryRSSI() const = 0;
 
   // The transmitted power level. This field is avaliable only for LE devices
   // that include this field in AD. It is avaliable and valid only during
   // discovery. If not during discovery, or TxPower wasn't reported, this
-  // method will return |kUnknownPower|.
-  virtual int16_t GetInquiryTxPower() const = 0;
+  // value will be absent.
+  virtual base::Optional<int8_t> GetInquiryTxPower() const = 0;
 
   // The ErrorCallback is used for methods that can fail in which case it
   // is called, in the success case the callback is simply not called.
@@ -328,7 +329,7 @@ class DEVICE_BLUETOOTH_EXPORT BluetoothDevice {
   // indicate a stronger signal.
   //
   // If the device isn't connected, then the ConnectionInfo struct passed into
-  // the callback will be populated with |kUnknownPower|.
+  // the callback will be populated with absent values.
   virtual void GetConnectionInfo(const ConnectionInfoCallback& callback) = 0;
 
   // Initiates a connection to the device, pairing first if necessary.