Reland fast tab closure behind a flag

chrome/browser/ui/views/frame/browser_view.cc

 // 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.
 
 #include "chrome/browser/ui/views/frame/browser_view.h"
 
 #include <algorithm>
 
 #include "base/auto_reset.h"
 #include "base/command_line.h"
@@ skipping 1797 matching lines @@
   // You cannot close a frame for which there is an active originating drag
   // session.
   if (tabstrip_ && !tabstrip_->IsTabStripCloseable())
     return false;
 
   // Give beforeunload handlers the chance to cancel the close before we hide
   // the window below.
   if (!browser_->ShouldCloseWindow())
     return false;
 
+  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.
     frame_->Hide();
     browser_->OnWindowClosing();
-    browser_->tab_strip_model()->CloseAllTabs();
+    if (fast_tab_closing_enabled)
+      browser_->tab_strip_model()->CloseAllTabs();
     return false;
-  } else if (!browser_->HasCompletedUnloadProcessing()) {
+  } else if (fast_tab_closing_enabled &&
+        !browser_->HasCompletedUnloadProcessing()) {
     // The browser needs to finish running unload handlers.
     // Hide the frame (so it appears to have closed immediately), and
     // the browser will call us back again when it is ready to close.
     frame_->Hide();
     return false;
   }
 
   // Empty TabStripModel, it's now safe to allow the Window to be closed.
   content::NotificationService::current()->Notify(
       chrome::NOTIFICATION_WINDOW_CLOSED,
@@ skipping 936 matching lines @@
     // The +1 in the next line creates a 1-px gap between icon and arrow tip.
     gfx::Point icon_bottom(0, location_icon_view->GetImageBounds().bottom() -
         LocationBarView::kIconInternalPadding + 1);
     ConvertPointToTarget(location_icon_view, this, &icon_bottom);
     gfx::Point infobar_top(0, infobar_container_->GetVerticalOverlap(NULL));
     ConvertPointToTarget(infobar_container_, this, &infobar_top);
     top_arrow_height = infobar_top.y() - icon_bottom.y();
   }
   return top_arrow_height;
 }