Implement create attribute API functions for BTLE.

device/bluetooth/bluetooth_adapter_mac.mm

Index: device/bluetooth/bluetooth_adapter_mac.mm
diff --git a/device/bluetooth/bluetooth_adapter_mac.mm b/device/bluetooth/bluetooth_adapter_mac.mm
index 3f55257390c64e076f2469e8102ef56d46f41d8a..931ac8430097d1d3ffbf6e7f0ec57c354d557506 100644
--- a/device/bluetooth/bluetooth_adapter_mac.mm
+++ b/device/bluetooth/bluetooth_adapter_mac.mm
@@ -207,6 +207,11 @@ void BluetoothAdapterMac::RegisterAdvertisement(
   error_callback.Run(BluetoothAdvertisement::ERROR_UNSUPPORTED_PLATFORM);
 }
 
+BluetoothLocalGattService* BluetoothAdapterMac::GetGattService(
+    const std::string& identifier) const {
+  return nullptr;
+}
+
 void BluetoothAdapterMac::ClassicDeviceFound(IOBluetoothDevice* device) {
   ClassicDeviceAdded(device);
 }