Reland fast tab closure behind a flag

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

Index: chrome/browser/ui/views/frame/browser_view.cc
diff --git a/chrome/browser/ui/views/frame/browser_view.cc b/chrome/browser/ui/views/frame/browser_view.cc
index dec5ac81df4d671cf1acc0910d7164f5e758827c..3d7ea91d3ecd35612030b63f1e67444bc713c964 100644
--- a/chrome/browser/ui/views/frame/browser_view.cc
+++ b/chrome/browser/ui/views/frame/browser_view.cc
@@ -1815,15 +1815,20 @@ bool BrowserView::CanClose() {
   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.