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

webrtc/build/webrtc.gni

Index: webrtc/build/webrtc.gni
diff --git a/webrtc/build/webrtc.gni b/webrtc/build/webrtc.gni
index c55f4230bdec9016d3c0d563fe775521906b7b3a..1c8bc23fcf9d019fb921de64f40e4ca20aacbda2 100644
--- a/webrtc/build/webrtc.gni
+++ b/webrtc/build/webrtc.gni
@@ -16,6 +16,9 @@ declare_args() {
   # Disable this to avoid building the Opus audio codec.
   rtc_include_opus = true
 
+  # Include the iLBC audio codec?
+  rtc_include_ilbc = false
+
   # Used to specify an external Jsoncpp include path when not compiling the
   # library that comes with WebRTC (i.e. rtc_build_json == 0).
   rtc_jsoncpp_root = "//third_party/jsoncpp/source/include"