Adding build switch for Opus that supports 120ms ptime.

webrtc/webrtc.gni

Index: webrtc/webrtc.gni
diff --git a/webrtc/webrtc.gni b/webrtc/webrtc.gni
index 53dcae2b7c862d9b6b3d21b986fa2c763d8050c6..4c34eae1922a0535f36b6ef5cb0f3ceac2d8490a 100644
--- a/webrtc/webrtc.gni
+++ b/webrtc/webrtc.gni
@@ -17,6 +17,10 @@ declare_args() {
   # Disable this to avoid building the Opus audio codec.
   rtc_include_opus = true
 
+  # Enable this if the Opus version upon which WebRTC is built supports direct
+  # encoding of 120 ms packets.
+  rtc_opus_support_120ms_ptime = false
+
   # Enable this to let the Opus audio codec change complexity on the fly.
   rtc_opus_variable_complexity = false