Added the Ash.Tab.TimeBetweenSwitchToExistingTabUserActions histogram to track time between tab swit

ash/shell/shell_delegate_impl.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_SHELL_SHELL_DELEGATE_IMPL_H_
 #define ASH_SHELL_SHELL_DELEGATE_IMPL_H_
 
 #include <string>
 
 #include "ash/shell_delegate.h"
@@ skipping 44 matching lines @@
   ash::SystemTrayDelegate* CreateSystemTrayDelegate() override;
   ash::UserWallpaperDelegate* CreateUserWallpaperDelegate() override;
   ash::SessionStateDelegate* CreateSessionStateDelegate() override;
   ash::AccessibilityDelegate* CreateAccessibilityDelegate() override;
   ash::NewWindowDelegate* CreateNewWindowDelegate() override;
   ash::MediaDelegate* CreateMediaDelegate() override;
   ui::MenuModel* CreateContextMenu(aura::Window* root_window,
                                    ash::ShelfItemDelegate* item_delegate,
                                    ash::ShelfItem* item) override;
   GPUSupport* CreateGPUSupport() override;
+  UserMetricsRecorderProxy* CreateUserMetricsRecorderProxy() override;
   base::string16 GetProductName() const override;
 
  private:
   // Used to update Launcher. Owned by main.
   WindowWatcher* watcher_;
 
   ShelfDelegateImpl* shelf_delegate_;
   content::BrowserContext* browser_context_;
   scoped_ptr<app_list::AppListViewDelegate> app_list_view_delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(ShellDelegateImpl);
 };
 
 }  // namespace shell
 }  // namespace ash
 
 #endif  // ASH_SHELL_SHELL_DELEGATE_IMPL_H_