Remove DeviceMonitorLinux::WillDestroyCurrentMessageLoop().

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 16275ad30c1d666a31ce8a83161b344352d68180..8393ae248e718456190407214981b3c404d30248 100644
--- a/device/test/test_device_client.h
+++ b/device/test/test_device_client.h
@@ -19,6 +19,8 @@ class UsbService;
 class TestDeviceClient : public DeviceClient {
  public:
   TestDeviceClient(scoped_refptr<base::SingleThreadTaskRunner> task_runner);
+
+  // Must be destroyed when tasks can still be posted to |task_runner|.
   ~TestDeviceClient() override;
 
   HidService* GetHidService() override;