Add VectorIconButton functionality to ImageButton.

ui/views/view_observer.h

 // Copyright 2016 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_VIEW_OBSERVER_H_
 #define UI_VIEWS_VIEW_OBSERVER_H_
 
 #include "ui/views/views_export.h"
 
+namespace ui {
+class NativeTheme;
+}
+
 namespace views {
 
 class View;
 
 // Observer can listen to various events on the Views.
 class VIEWS_EXPORT ViewObserver {
  public:
   // Called when |child| is added to its parent. Invoked on the parent which has
   // the |child| added to it.
   virtual void OnChildViewAdded(View* child) {}
 
   // Called when |child| is removed from its |parent|.
   virtual void OnChildViewRemoved(View* child, View* parent) {}
 
   // Called when View::SetVisible() is called with a new value.
   // The |view| may still be hidden at this point.
   virtual void OnViewVisibilityChanged(View* view) {}
 
   virtual void OnViewEnabledChanged(View* view) {}
 
   virtual void OnViewBoundsChanged(View* view) {}
 
   // Invoked whenever a child is moved to another index. This is called on the
   // parent view. |view| is the child view being moved.
   virtual void OnChildViewReordered(View* view) {}
 
+  virtual void OnThemeChanged(View* view) {}
+
+  virtual void OnNativeThemeChanged(View* view, const ui::NativeTheme* theme) {}
+
  protected:
   virtual ~ViewObserver() {}
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_VIEW_OBSERVER_H_