Refactor ViewsDelegate singleton

ui/views/views_delegate.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 UI_VIEWS_VIEWS_DELEGATE_H_
 #define UI_VIEWS_VIEWS_DELEGATE_H_
 
 #include <string>
 
 #if defined(OS_WIN)
@@ skipping 42 matching lines @@
 #endif
 
 namespace internal {
 class NativeWidgetDelegate;
 }
 
 // ViewsDelegate is an interface implemented by an object using the views
 // framework. It is used to obtain various high level application utilities
 // and perform some actions such as window placement saving.
 //
-// The embedding app must set views_delegate to assign its ViewsDelegate
-// implementation.
+// The embedding app must set the ViewsDelegate instance by instantiating an
+// implementation of ViewsDelegate (the constructor will set the instance).
 class VIEWS_EXPORT ViewsDelegate {
  public:
 #if defined(OS_WIN)
   enum AppbarAutohideEdge {
     EDGE_TOP    = 1 << 0,
     EDGE_LEFT   = 1 << 1,
     EDGE_BOTTOM = 1 << 2,
     EDGE_RIGHT  = 1 << 3,
   };
 #endif
 
-  ViewsDelegate();
   virtual ~ViewsDelegate();
 
+  // Returns the ViewsDelegate instance if there is one, or nullptr otherwise.
+  static ViewsDelegate* GetInstance();
+
   // Saves the position, size and "show" state for the window with the
   // specified name.
   virtual void SaveWindowPlacement(const Widget* widget,
                                    const std::string& window_name,
                                    const gfx::Rect& bounds,
                                    ui::WindowShowState show_state);
 
   // Retrieves the saved position and size and "show" state for the window with
   // the specified name.
   virtual bool GetSavedWindowPlacement(const Widget* widget,
@@ skipping 64 matching lines @@
   // not changed, |callback| is never run.
   //
   // The return value is a bitmask of AppbarAutohideEdge.
   virtual int GetAppbarAutohideEdges(HMONITOR monitor,
                                      const base::Closure& callback);
 #endif
 
   // Returns a blocking pool task runner given a TaskRunnerType.
   virtual scoped_refptr<base::TaskRunner> GetBlockingPoolTaskRunner();
 
-  // The active ViewsDelegate used by the views system.
-  static ViewsDelegate* views_delegate;
+ protected:
+  ViewsDelegate();
 
  private:
   scoped_ptr<ViewsTouchEditingControllerFactory> views_tsc_factory_;
 
 #if defined(USE_AURA)
   scoped_ptr<TouchSelectionMenuRunnerViews> touch_selection_menu_runner_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(ViewsDelegate);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_VIEWS_DELEGATE_H_