Adds a WebRTC browser_test with opus dtx enabled.

chrome/browser/media/webrtc_video_quality_browsertest.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stddef.h>
 
 #include "base/base64.h"
 #include "base/command_line.h"
 #include "base/environment.h"
 #include "base/files/file.h"
@@ skipping 280 matching lines @@
             embedded_test_server()->GetURL(kMainWebrtcTestHtmlPage),
             test_config_.constraints);
     content::WebContents* right_tab =
         OpenPageAndGetUserMediaInNewTabWithConstraints(
             embedded_test_server()->GetURL(kCapturingWebrtcHtmlPage),
             test_config_.constraints);
 
     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.
     int polling_interval_msec = 1000;
 
     EXPECT_TRUE(test::PollingWaitUntil("doneFrameCapturing()", "done-capturing",
                                        right_tab, polling_interval_msec));
 
     HangUp(left_tab);
 
@@ skipping 64 matching lines @@
   if (!base::FeatureList::IsEnabled(content::kWebRtcH264WithOpenH264FFmpeg)) {
     LOG(WARNING) << "Run-time feature WebRTC-H264WithOpenH264FFmpeg disabled. "
         "Skipping WebRtcVideoQualityBrowserTest.MANUAL_TestVideoQualityH264 "
         "(test \"OK\")";
     return;
   }
   TestVideoQuality("H264");
 }
 
 #endif  // BUILDFLAG(RTC_USE_H264)