Fix race condition in WebRtcInternalsBrowserTest.

SetUpOnMainThread is too late to mutate the command-line. By then, other
threads have started and may be reading from it. Instead, use
SetUpCommandLine.

This should fix the following TSan flake:
https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8917832371169415328/+/steps/content_browsertests/0/logs/Flaky_failure:_WebRtcInternalsBrowserTest.UpdateAllPeerConnections__status_FAILURE_ON_EXIT_SUCCESS_/0

Change-Id: Ibf819a3247b290b9d583d25dc0111881868c4a74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1541497
Auto-Submit: David Benjamin <davidben@chromium.org>
Reviewed-by: Alexander Potapenko <glider@chromium.org>
Reviewed-by: Christian Fremerey <chfremer@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646407}

content/browser/webrtc/webrtc_internals_browsertest.cc

@@ -155,9 +155,11 @@ class MAYBE_WebRtcInternalsBrowserTest: public ContentBrowserTest {
   MAYBE_WebRtcInternalsBrowserTest() {}
   ~MAYBE_WebRtcInternalsBrowserTest() override {}
 
+  void SetUpCommandLine(base::CommandLine* command_line) override {
+    command_line->AppendSwitch(switches::kUseFakeUIForMediaStream);
+  }
+
   void SetUpOnMainThread() override {
-    base::CommandLine::ForCurrentProcess()->AppendSwitch(
-        switches::kUseFakeUIForMediaStream);
     ASSERT_TRUE(base::CommandLine::ForCurrentProcess()->HasSwitch(
         switches::kUseFakeDeviceForMediaStream));
   }