Reland fast tab closure behind a flag

chrome/browser/ui/cocoa/browser_window_controller.mm

Index: chrome/browser/ui/cocoa/browser_window_controller.mm
diff --git a/chrome/browser/ui/cocoa/browser_window_controller.mm b/chrome/browser/ui/cocoa/browser_window_controller.mm
index 2aac9f40ba24aa1bd415f0c69b58991a43e60486..608ac875042ecf33b2c84f6fa9c2fdccea864dc1 100644
--- a/chrome/browser/ui/cocoa/browser_window_controller.mm
+++ b/chrome/browser/ui/cocoa/browser_window_controller.mm
@@ -597,15 +597,20 @@ enum {
   // 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.