Add smoke test for UsbService::GetDevices.

device/test/test_device_client.h

Index: device/test/test_device_client.h
diff --git a/device/test/test_device_client.h b/device/test/test_device_client.h
index ce8cc2d16358da299083471a7a8e62d955ed5558..d4cd62790145d33cd1df3798cce6bb2b3c764d6d 100644
--- a/device/test/test_device_client.h
+++ b/device/test/test_device_client.h
@@ -21,10 +21,10 @@ class TestDeviceClient : public DeviceClient {
   TestDeviceClient(scoped_refptr<base::SingleThreadTaskRunner> task_runner);
   ~TestDeviceClient() override;
 
- private:
   HidService* GetHidService() override;
   UsbService* GetUsbService() override;
 
+ private:
   std::unique_ptr<HidService> hid_service_;
   std::unique_ptr<UsbService> usb_service_;
   scoped_refptr<base::SingleThreadTaskRunner> blocking_task_runner_;