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..ed8894f3c98f914e9c8cba30117d026a5e690cd4 100644
--- a/webrtc/build/webrtc.gni
+++ b/webrtc/build/webrtc.gni
@@ -103,6 +103,14 @@ declare_args() {
   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"