Allow device/bluetooth to build on the macOS 10.13 SDK

device/bluetooth/bluetooth_low_energy_device_mac.mm

Index: device/bluetooth/bluetooth_low_energy_device_mac.mm
diff --git a/device/bluetooth/bluetooth_low_energy_device_mac.mm b/device/bluetooth/bluetooth_low_energy_device_mac.mm
index e80b9c5192c18e7fee04b2b9193c473fd7d1367b..1e25ff41326e93ba4a4cd90c7abf1a91906eeaab 100644
--- a/device/bluetooth/bluetooth_low_energy_device_mac.mm
+++ b/device/bluetooth/bluetooth_low_energy_device_mac.mm
@@ -20,6 +20,18 @@
 #include "device/bluetooth/bluetooth_remote_gatt_descriptor_mac.h"
 #include "device/bluetooth/bluetooth_remote_gatt_service_mac.h"
 
+// Remove when Chrome no longer supports 10.12.
+#if defined(MAC_OS_X_VERSION_10_13)
+
+// In the 10.13 SDK, CBPeripheral became a subclass of CBPeer, which defines
+// -[CBPeer identifier] as partially available. Pretend it still exists on
+// CBPeripheral. At runtime the implementation on CBPeer will be invoked.
+@interface CBPeripheral (HighSierraSDK)
+@property(readonly, nonatomic) NSUUID* identifier;
+@end
+
+#endif  // MAC_OS_X_VERSION_10_13
+
 namespace device {
 
 BluetoothLowEnergyDeviceMac::BluetoothLowEnergyDeviceMac(