Disable audio output for WebRTC content browser tests.

This intends to address a series of flakes caused
by problems with Pulse Audio on some bots.

Bug: 1047655
Change-Id: Iade028ea431cdaa8df3255a246c5f23c0365ff9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2066911
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Auto-Submit: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744089}

content/browser/webrtc/webrtc_content_browsertest_base.cc

@@ -31,6 +31,12 @@ void WebRtcContentBrowserTestBase::SetUpCommandLine(
     base::CommandLine* command_line) {
   base::CommandLine::ForCurrentProcess()->AppendSwitch(
       switches::kEnforceWebRtcIPPermissionCheck);
+#if defined(OS_LINUX)
+  // Due to problems with PulseAudio failing to start, use a fake audio
+  // stream. crbug.com/1047655#c70
+  base::CommandLine::ForCurrentProcess()->AppendSwitch(
+      switches::kDisableAudioOutput);
+#endif
 
   // Loopback interface is the non-default local address. They should only be in
   // the candidate list if the ip handling policy is "default" AND the media