Reland fast tab closure behind a flag

chrome/browser/ui/cocoa/browser_window_controller.mm

 // Copyright 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.
 
 #import "chrome/browser/ui/cocoa/browser_window_controller.h"
 
 #include <cmath>
 #include <numeric>
 
 #include "base/command_line.h"
@@ skipping 579 matching lines @@
   // Give beforeunload handlers the chance to cancel the close before we hide
   // the window below.
   if (!browser_->ShouldCloseWindow())
     return NO;
 
   // saveWindowPositionIfNeeded: only works if we are the last active
   // window, but orderOut: ends up activating another window, so we
   // have to save the window position before we call orderOut:.
   [self saveWindowPositionIfNeeded];
 
+  bool fast_tab_closing_enabled =
+      CommandLine::ForCurrentProcess()->HasSwitch(switches::kEnableFastUnload);
+
   if (!browser_->tab_strip_model()->empty()) {
     // Tab strip isn't empty.  Hide the frame (so it appears to have closed
     // immediately) and close all the tabs, allowing the renderers to shut
     // down. When the tab strip is empty we'll be called back again.
     [[self window] orderOut:self];
     browser_->OnWindowClosing();
-    browser_->tab_strip_model()->CloseAllTabs();
+    if (fast_tab_closing_enabled)
+      browser_->tab_strip_model()->CloseAllTabs();
     return NO;
-  } else if (!browser_->HasCompletedUnloadProcessing()) {
+  } else if (fast_tab_closing_enabled &&
+        !browser_->HasCompletedUnloadProcessing()) {
     // The browser needs to finish running unload handlers.
     // Hide the window (so it appears to have closed immediately), and
     // the browser will call us back again when it is ready to close.
     [[self window] orderOut:self];
     return NO;
   }
 
   // the tab strip is empty, it's ok to close the window
   return YES;
 }
@@ skipping 1650 matching lines @@
 
 - (BOOL)supportsBookmarkBar {
   return [self supportsWindowFeature:Browser::FEATURE_BOOKMARKBAR];
 }
 
 - (BOOL)isTabbedWindow {
   return browser_->is_type_tabbed();
 }
 
 @end  // @implementation BrowserWindowController(WindowType)