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 73eba37f94c7ee1ae02ad9bdb24648e68dbd8b82..c490bf119de2272a7715108527c40ce7991df9d3 100644
--- a/chrome/browser/ui/cocoa/browser_window_controller.mm
+++ b/chrome/browser/ui/cocoa/browser_window_controller.mm
@@ -596,12 +596,24 @@ 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();
+    if (fast_tab_closing_enabled)
+      browser_->tab_strip_model()->CloseAllTabs();
+    return NO;
+  } 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;
   }