Avoid changing enabled features while the browser is already running.

chrome/browser/ui/extensions/hosted_app_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 <string>
 
 #include "base/command_line.h"
 #include "base/compiler_specific.h"
 #include "base/strings/stringprintf.h"
 #include "base/test/scoped_feature_list.h"
@@ skipping 185 matching lines @@
   // Navigate to different origin; the location bar should now be visible.
   NavigateAndCheckForLocationBar(
       app_browser_, "http://www.foo.com/blah", true);
 }
 
 class HostedAppVsTdiTest : public HostedAppTest {
  public:
   HostedAppVsTdiTest() {}
   ~HostedAppVsTdiTest() override {}
 
+  void SetUp() override {
+    scoped_feature_list_.InitAndEnableFeature(features::kTopDocumentIsolation);
+    HostedAppTest::SetUp();
+  }
+
   void SetUpOnMainThread() override {
-    scoped_feature_list_.InitAndEnableFeature(features::kTopDocumentIsolation);
     HostedAppTest::SetUpOnMainThread();
     host_resolver()->AddRule("*", "127.0.0.1");
     ASSERT_TRUE(embedded_test_server()->Start());
   }
 
  private:
   base::test::ScopedFeatureList scoped_feature_list_;
 
   DISALLOW_COPY_AND_ASSIGN(HostedAppVsTdiTest);
 };
@@ skipping 125 matching lines @@
   } else {
     // TODO(lukasza): https://crbug.com/718516: Process policy is not
     // well-defined / settled wrt relationship between 1) hosted apps and 2)
     // same-site web content outside of hosted app's extent.  When this test was
     // authored --site-per-process would put |app| in a separate renderer
     // process from |diff_dir| and |same_site|, even though such process
     // placement can be problematic (if |app| tries to synchronously script
     // |diff_dir| and/or |same_site|).
   }
 }