[Chromecast] Replace architecure-based conditionals with flag.

media/midi/midi.gyp

 # 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.
 
 {
   'variables': {
     'chromium_code': 1,
     'conditions': [
-      ['(OS=="linux" or OS=="freebsd" or OS=="solaris") and (embedded!=1 or (chromecast==1 and target_arch!="arm"))', {
+      ['(OS=="linux" or OS=="freebsd" or OS=="solaris") and (chromecast==0 or is_cast_desktop_build==1)', {
         'use_alsa%': 1,
       }, {
         'use_alsa%': 0,
       }],
     ],
     # Source files that are used in production code for Android, and in tests
     # for others.
     'usb_midi_sources': [
       'midi_manager_usb.cc',
       'midi_manager_usb.h',
@@ skipping 222 matching lines @@
             '../../build/isolate.gypi',
           ],
           'sources': [
             'midi_unittests.isolate',
           ],
         },
       ],
     }],
   ],
 }