Adds a WebRTC browser_test with opus dtx enabled.

chrome/browser/media/webrtc_video_quality_browsertest.cc

Index: chrome/browser/media/webrtc_video_quality_browsertest.cc
diff --git a/chrome/browser/media/webrtc_video_quality_browsertest.cc b/chrome/browser/media/webrtc_video_quality_browsertest.cc
index 566265371e4d52b54fe2d70b570d1e63c9e40183..64909476d837b262dc0d71766557c117dc10092d 100644
--- a/chrome/browser/media/webrtc_video_quality_browsertest.cc
+++ b/chrome/browser/media/webrtc_video_quality_browsertest.cc
@@ -298,7 +298,11 @@ class WebRtcVideoQualityBrowserTest : public WebRtcTestBase,
     SetupPeerconnectionWithLocalStream(left_tab);
     SetupPeerconnectionWithLocalStream(right_tab);
 
-    NegotiateCall(left_tab, right_tab, video_codec);
+    if (!video_codec.empty()) {
+      SetDefaultVideoCodec(left_tab, video_codec);
+      SetDefaultVideoCodec(right_tab, video_codec);
+    }
+    NegotiateCall(left_tab, right_tab);
 
     // Poll slower here to avoid flooding the log with messages: capturing and
     // sending frames take quite a bit of time.