Reland fast tab closure behind a flag

chrome/browser/ui/tab_contents/core_tab_helper.h

Index: chrome/browser/ui/tab_contents/core_tab_helper.h
diff --git a/chrome/browser/ui/tab_contents/core_tab_helper.h b/chrome/browser/ui/tab_contents/core_tab_helper.h
index 69aafd23fd4383d60ec4642d3d749c1b72d94e20..c8e67828fdc6e825245010f60613ad3ffc8e09d2 100644
--- a/chrome/browser/ui/tab_contents/core_tab_helper.h
+++ b/chrome/browser/ui/tab_contents/core_tab_helper.h
@@ -31,6 +31,14 @@ class CoreTabHelper : public content::WebContentsObserver,
   // cancels a window close via another tab's beforeunload dialog.
   void OnCloseCanceled();
 
+  // Set the time during close when unload is started. Normally, this is set
+  // after the beforeunload dialog. However, for a window close, it is set
+  // after all the beforeunload dialogs have finished.
+  void OnUnloadStarted();
+
+  // Set the time during close when the tab is no longer visible.
+  void OnUnloadDetachedStarted();
+
   CoreTabHelperDelegate* delegate() const { return delegate_; }
   void set_delegate(CoreTabHelperDelegate* d) { delegate_ = d; }
 
@@ -64,6 +72,9 @@ class CoreTabHelper : public content::WebContentsObserver,
   // The time when onbeforeunload ended.
   base::TimeTicks before_unload_end_time_;
 
+  // The time when the tab was removed from view during close.
+  base::TimeTicks unload_detached_start_time_;
+
   DISALLOW_COPY_AND_ASSIGN(CoreTabHelper);
 };