Add a gyp/gn variable for whether to use iLBC or not

webrtc/build/webrtc.gni

 # Copyright (c) 2014 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.
 
 import("//build/config/arm.gni")
 import("//build/config/mips.gni")
@@ skipping 85 matching lines @@
   # and FFmpeg have to be correctly enabled separately.
   # - use_openh264=true is required for OpenH264 targets to be defined.
   # - ffmpeg_branding="Chrome" is one way to support H.264 decoding in FFmpeg.
   #   FFmpeg can be built with/without H.264 support, see 'ffmpeg_branding'.
   #   Without it, it compiles but H264DecoderImpl fails to initialize.
   # CHECK THE OPENH264, FFMPEG AND H.264 LICENSES/PATENTS BEFORE BUILDING.
   # http://www.openh264.org, https://www.ffmpeg.org/
   use_third_party_h264 = false  # TODO(hbos): To be used in follow-up CL(s).
 }
 
+# A second declare_args block, so that declarations within it can
+# depend on the possibly overridden variables in the first
+# declare_args block.
+declare_args() {
+  # Include the iLBC audio codec?
+  rtc_include_ilbc = !(build_with_chromium || build_with_mozilla)
+}
+
 # Make it possible to provide custom locations for some libraries (move these
 # up into declare_args should we need to actually use them for the GN build).
 rtc_libvpx_dir = "//third_party/libvpx_new"
 rtc_libyuv_dir = "//third_party/libyuv"
 rtc_opus_dir = "//third_party/opus"