Adding build switch for Opus that supports 120ms ptime.

BUG=webrtc:7097

TEST=Set "ptime=120", try WebRTC calls over custom build Chromium with and without Opus 120ms. Try both Chromium w <-> Chromium w and Chromium w <-> Chromium w/o

Review-Url: https://codereview.webrtc.org/2668633004
Cr-Commit-Position: refs/heads/master@{#16408}
diff --git a/webrtc/webrtc.gni b/webrtc/webrtc.gni
index 53dcae2..4c34eae 100644
--- a/webrtc/webrtc.gni
+++ b/webrtc/webrtc.gni
@@ -17,6 +17,10 @@
   # 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