Avoid changing enabled features while the browser is already running.

content/public/test/browser_test_base.cc

Index: content/public/test/browser_test_base.cc
diff --git a/content/public/test/browser_test_base.cc b/content/public/test/browser_test_base.cc
index 9526a3927e82bbcaf0e6bcdea5e131530abbcb63..577bc75e2bdc016a2709754cef60c03d83d4d171 100644
--- a/content/public/test/browser_test_base.cc
+++ b/content/public/test/browser_test_base.cc
@@ -250,6 +250,7 @@ void BrowserTestBase::SetUp() {
   // Need to wipe feature list clean, since BrowserMain calls
   // FeatureList::SetInstance, which expects no instance to exist.
   base::FeatureList::ClearInstanceForTesting();
+  base::FeatureList::BlockOverridingInstanceForTesting(true);
 
   base::Closure* ui_task =
       new base::Closure(
@@ -266,6 +267,7 @@ void BrowserTestBase::SetUp() {
   EXPECT_EQ(expected_exit_code_, ContentMain(*GetContentMainParams()));
 #endif
   TearDownInProcessBrowserTestFixture();
+  base::FeatureList::BlockOverridingInstanceForTesting(false);
 }
 
 void BrowserTestBase::TearDown() {