Add a .gyp option to use dummy audio file devices.

webrtc/build/common.gypi

 # Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 # This file contains common settings for building WebRTC components.
 
@@ skipping 155 matching lines @@
 
     # Enable this to prevent extern symbols from being hidden on iOS builds.
     # The chromium settings we inherit hide symbols by default on Release
     # builds. We want our symbols to be visible when distributing WebRTC via
     # static libraries to avoid linker warnings.
     'ios_override_visibility%': 0,
 
     # Determines whether QUIC code will be built.
     'use_quic%': 0,
 
+    # By default, use normal platform audio support or dummy audio, but don't
+    # use file-based audio playout and record.
+    'use_dummy_audio_file_devices%': 0,
+
     'conditions': [
       # Enable this to build OpenH264 encoder/FFmpeg decoder. This is supported
       # on all platforms except Android and iOS. Because FFmpeg can be built
       # with/without H.264 support, |ffmpeg_branding| has to separately be set
       # to a value that includes H.264, for example "Chrome". If FFmpeg is built
       # without H.264, compilation succeeds but |H264DecoderImpl| fails to
       # initialize. See also: |rtc_initialize_ffmpeg|.
       # CHECK THE OPENH264, FFMPEG AND H.264 LICENSES/PATENTS BEFORE BUILDING.
       # http://www.openh264.org, https://www.ffmpeg.org/
       ['proprietary_codecs==1 and OS!="android" and OS!="ios"', {
@@ skipping 412 matching lines @@
         }],
         ['OS=="freebsd"', {
           'defines': [
             'FREEBSD',
           ],
         }],
       ],
     },
   }, # target_defaults
 }