Fix TSan flake from ContentMockCertVerifier::set_default_result

This seems to be flaky on TSan builds when set_default_result is called
from SetUpOnMainThread(). This should prevent calling it in this case.

Bug: 1159734
Change-Id: Id08167574874af6bb7aab5dd03df6622d062b7a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618393
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Auto-Submit: Clark DuVall <cduvall@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841684}

content/public/test/content_mock_cert_verifier.cc

@@ -5,6 +5,7 @@
 #include "content/public/test/content_mock_cert_verifier.h"
 
 #include "base/command_line.h"
+#include "base/feature_list.h"
 #include "content/public/browser/network_service_instance.h"
 #include "content/public/common/content_switches.h"
 #include "content/public/common/network_service_util.h"
@@ -26,12 +27,18 @@ void ContentMockCertVerifier::CertVerifier::set_default_result(
     int default_result) {
   verifier_->set_default_result(default_result);
 
-  // Set the default result as a flag in case the FeatureList has not been
-  // initialized yet and we don't know if network service will run out of
-  // process.
-  base::CommandLine::ForCurrentProcess()->AppendSwitchASCII(
-      switches::kMockCertVerifierDefaultResultForTesting,
-      base::NumberToString(default_result));
+  // If set_default_result is called before the FeatureList is available, add
+  // the command line flag since the network service may be running out of
+  // process. We don't want to set the command line flag otherwise since it can
+  // cause TSan errors.
+  if (base::FeatureList::GetInstance() == nullptr) {
+    // Set the default result as a flag in case the FeatureList has not been
+    // initialized yet and we don't know if network service will run out of
+    // process.
+    base::CommandLine::ForCurrentProcess()->AppendSwitchASCII(
+        switches::kMockCertVerifierDefaultResultForTesting,
+        base::NumberToString(default_result));
+  }
 
   if (IsInProcessNetworkService())
     return;