Reland fast tab closure behind a flag

chrome/browser/ui/gtk/browser_window_gtk.cc

Index: chrome/browser/ui/gtk/browser_window_gtk.cc
diff --git a/chrome/browser/ui/gtk/browser_window_gtk.cc b/chrome/browser/ui/gtk/browser_window_gtk.cc
index f343c44ecf2e4b762abb3a6a910fe559416d8ae5..0827d8e2a522d3bfea19a23bab16927753c36d6c 100644
--- a/chrome/browser/ui/gtk/browser_window_gtk.cc
+++ b/chrome/browser/ui/gtk/browser_window_gtk.cc
@@ -1443,15 +1443,20 @@ bool BrowserWindowGtk::CanClose() const {
   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 window (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.
     gtk_widget_hide(GTK_WIDGET(window_));
     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 window (so it appears to have closed immediately), and
     // the browser will call us back again when it is ready to close.