Reland fast tab closure behind a flag

chrome/browser/unload_browsertest.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.
 
 #if defined(OS_POSIX)
 #include <signal.h>
 #endif
 
 #include "base/command_line.h"
 #include "base/logging.h"
@@ skipping 395 matching lines @@
       content::NOTIFICATION_WEB_CONTENTS_DESTROYED,
       content::NotificationService::AllSources());
   chrome::CloseTab(browser());
   tab_close_observer.Wait();
 
   CheckTitle("only_one_unload");
 }
 
 class FastUnloadTest : public UnloadTest {
  public:
+  virtual void SetUpCommandLine(CommandLine* command_line) OVERRIDE {
+    UnloadTest::SetUpCommandLine(command_line);
+    command_line->AppendSwitch(switches::kEnableFastUnload);
+  }
+
   virtual void SetUpInProcessBrowserTestFixture() OVERRIDE {
     ASSERT_TRUE(test_server()->Start());
   }
 
   virtual void TearDownInProcessBrowserTestFixture() OVERRIDE {
     test_server()->Stop();
   }
 
   GURL GetUrl(const std::string& name) {
     return GURL(test_server()->GetURL(
@@ skipping 184 matching lines @@
   content::WindowedNotificationObserver window_observer(
         chrome::NOTIFICATION_BROWSER_CLOSED,
         content::NotificationService::AllSources());
   chrome::CloseWindow(browser());
   CrashTab(beforeunload_contents);
   window_observer.Wait();
 }
 
 // TODO(ojan): Add tests for unload/beforeunload that have multiple tabs
 // and multiple windows.