Properly set loopback device ID for system audio capture.

content/content_browser.gypi

 # Copyright (c) 2012 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': {
-    'use_cras%': 0,
-  },
   'dependencies': [
     'browser/speech/proto/speech_proto.gyp:speech_proto',
     '../base/base.gyp:base_static',
     '../crypto/crypto.gyp:crypto',
     '../google_apis/google_apis.gyp:google_apis',
     '../net/net.gyp:net',
     '../skia/skia.gyp:skia',
     '../sql/sql.gyp:sql',
     '../third_party/re2/re2.gyp:re2',
     '../third_party/zlib/zlib.gyp:zip',
@@ skipping 1345 matching lines @@
         '../build/linux/system.gyp:dbus',
         '../build/linux/system.gyp:fontconfig',
         '../dbus/dbus.gyp:dbus',
       ],
     }],
     ['use_x11==1', {
       'dependencies': [
         '../build/linux/system.gyp:x11',
       ],
     }],
-    ['use_cras==1', {
-      'defines': [
-        'USE_CRAS',
-      ],
-    }],
     ['use_pango==1', {
       'dependencies': [
         '../build/linux/system.gyp:pangocairo',
       ],
     }],
     ['OS=="android"', {
       'dependencies': [
         '../media/media.gyp:media',
         'content.gyp:content_jni_headers',
       ],
@@ skipping 136 matching lines @@
         '../third_party/speex/speex.gyp:libspeex',
       ],
     }],
     ['linux_use_libgps==1', {
       'dependencies': [
         '../build/linux/system.gyp:libgps',
       ],
     }],
   ],
 }