[MD] Implement incognito colors as a NativeTheme (for Aura).

ui/views/widget/widget.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_WIDGET_WIDGET_H_
 #define UI_VIEWS_WIDGET_WIDGET_H_
 
 #include <set>
 #include <stack>
 #include <vector>
@@ skipping 543 matching lines @@
   // Returns whether the Widget is visible to the user.
   virtual bool IsVisible() const;
 
   // Returns the ThemeProvider that provides theme resources for this Widget.
   virtual ui::ThemeProvider* GetThemeProvider() const;
 
   ui::NativeTheme* GetNativeTheme() {
     return const_cast<ui::NativeTheme*>(
         const_cast<const Widget*>(this)->GetNativeTheme());
   }
-  const ui::NativeTheme* GetNativeTheme() const;
+  virtual const ui::NativeTheme* GetNativeTheme() const;
 
   // Returns the FocusManager for this widget.
   // Note that all widgets in a widget hierarchy share the same focus manager.
   FocusManager* GetFocusManager();
   const FocusManager* GetFocusManager() const;
 
   // Returns the ui::InputMethod for this widget.
   ui::InputMethod* GetInputMethod();
 
   // Starts a drag operation for the specified view. This blocks until the drag
@@ skipping 384 matching lines @@
   bool movement_disabled_;
 
   ScopedObserver<ui::NativeTheme, ui::NativeThemeObserver> observer_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(Widget);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_WIDGET_H_