Avoid changing enabled features while the browser is already running.

chrome/test/base/in_process_browser_test.cc

 // Copyright (c) 2012 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 "chrome/test/base/in_process_browser_test.h"
 
 #include "base/auto_reset.h"
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/feature_list.h"
@@ skipping 173 matching lines @@
 #endif
 }
 
 InProcessBrowserTest::~InProcessBrowserTest() {
 }
 
 void InProcessBrowserTest::SetUp() {
   // Browser tests will create their own g_browser_process later.
   DCHECK(!g_browser_process);
 
-  // Clear the FeatureList instance from base/test/test_suite.cc. Since this is
-  // a browser test, a FeatureList will be registered as part of normal browser
-  // start up in ChromeBrowserMainParts::SetupMetricsAndFieldTrials().
-  base::FeatureList::ClearInstanceForTesting();
-
   base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
 
   // Auto-reload breaks many browser tests, which assume error pages won't be
   // reloaded out from under them. Tests that expect or desire this behavior can
   // append switches::kEnableOfflineAutoReload, which will override the disable
   // here.
   command_line->AppendSwitch(switches::kDisableOfflineAutoReload);
 
   // Allow subclasses to change the command line before running any tests.
   SetUpCommandLine(command_line);
@@ skipping 414 matching lines @@
   // On the Mac, this eventually reaches
   // -[BrowserWindowController windowWillClose:], which will post a deferred
   // -autorelease on itself to ultimately destroy the Browser object. The line
   // below is necessary to pump these pending messages to ensure all Browsers
   // get deleted.
   content::RunAllPendingInMessageLoop();
   delete autorelease_pool_;
   autorelease_pool_ = NULL;
 #endif
 }