Avoid changing enabled features while the browser is already running.

chrome/browser/task_manager/providers/web_contents/subframe_task_browsertest.cc

 // Copyright 2015 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 "base/command_line.h"
 #include "base/macros.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/test/scoped_feature_list.h"
 #include "chrome/browser/task_manager/mock_web_contents_task_manager.h"
 #include "chrome/browser/ui/browser.h"
@@ skipping 188 matching lines @@
               unresponsive_task == subframe_task_2);
 }
 
 // A test for top document isolation and how subframes show up in the task
 // manager as SubframeTasks.
 class SubframeTaskTDIBrowserTest : public InProcessBrowserTest {
  public:
   SubframeTaskTDIBrowserTest() {}
   ~SubframeTaskTDIBrowserTest() override {}
 
+  void SetUp() override {
+    scoped_feature_list_.InitAndEnableFeature(features::kTopDocumentIsolation);
+    InProcessBrowserTest::SetUp();
+  }
+
   void SetUpOnMainThread() override {
-    scoped_feature_list_.InitAndEnableFeature(features::kTopDocumentIsolation);
     host_resolver()->AddRule("*", "127.0.0.1");
     ASSERT_TRUE(embedded_test_server()->InitializeAndListen());
     content::SetupCrossSiteRedirector(embedded_test_server());
     embedded_test_server()->StartAcceptingConnections();
   }
 
   void NavigateTo(const char* page_url) const {
     ui_test_utils::NavigateToURL(browser(),
                                  embedded_test_server()->GetURL(page_url));
   }
@@ skipping 45 matching lines @@
                                       base::UTF8ToUTF16(new_title));
   EXPECT_TRUE(ExecuteScript(web_contents, script));
   base::string16 actual_title = title_watcher.WaitAndGetTitle();
   EXPECT_EQ(new_title, base::UTF16ToUTF8(actual_title));
 
   EXPECT_EQ(PrefixExpectedTabTitle(new_title), main_task->title());
   EXPECT_EQ(PrefixExpectedTDIRowTitle(new_title), tdi_task->title());
 }
 
 }  // namespace task_manager