Add is_chromecast to GN build.

media/midi/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//media/media_options.gni")
 import("//testing/test.gni")
 
 if (is_android) {
   # For generate_jni().
   import("//build/config/android/rules.gni")
 }
 
 # Common configuration for targets in the media/midi directory.
 config("midi_config") {
-  if (use_alsa) {
+  if (use_alsa && use_udev) {
     defines = [
       "USE_ALSA",
       "USE_UDEV",
     ]
   }
 
   visibility = [ ":*" ]
 }
 
 # USB MIDI specific source files that are used in Android port, and unit tests.
@@ skipping 162 matching lines @@
   }
 
   if (use_alsa && use_udev) {
     sources += [ "midi_manager_alsa_unittest.cc" ]
   }
 
   if (use_x11) {
     deps += [ "//tools/xdisplaycheck" ]
   }
 }