Fix for clang build warnings: undefined variables if !USE_UDEV.

media/midi/midi_manager_alsa.cc

Index: media/midi/midi_manager_alsa.cc
diff --git a/media/midi/midi_manager_alsa.cc b/media/midi/midi_manager_alsa.cc
index 4b459fdf3658f6f372effb9f42694da1806bc17b..9015be9913033bb54d4b78b2b7e2640c567c8920 100644
--- a/media/midi/midi_manager_alsa.cc
+++ b/media/midi/midi_manager_alsa.cc
@@ -43,6 +43,7 @@ const size_t kSendBufferSize = 256;
 const int kMinimumClientIdForCards = 16;
 
 // udev key constants.
+#if defined(USE_UDEV)
 const char kSoundClass[] = "sound";
 const char kIdVendor[] = "ID_VENDOR";
 const char kIdVendorEnc[] = "ID_VENDOR_ENC";
@@ -55,6 +56,7 @@ const char kSysattrModel[] = "model";
 const char kIdBus[] = "ID_BUS";
 const char kIdPath[] = "ID_PATH";
 const char kUsbInterfaceNum[] = "ID_USB_INTERFACE_NUM";
+#endif  // defined(USE_UDEV)
 
 // ALSA constants.
 const char kAlsaHw[] = "hw";