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;
 
 // ViewObserver is used to observe changes to a View. The first argument to all
 // ViewObserver functions is the View the observer was added to.
 class VIEWS_EXPORT ViewObserver {
  public:
   // Called when |child| is added as a child to |observed_view|.
   virtual void OnChildViewAdded(View* observed_view, View* child) {}
@@ skipping 11 matching lines @@
   // Called when the bounds of |observed_view| change.
   virtual void OnViewBoundsChanged(View* observed_view) {}
 
   // Called when a child is reordered among its siblings, specifically
   // View::ReorderChildView() is called on |observed_view|.
   virtual void OnChildViewReordered(View* observed_view, View* child) {}
 
   // Called from ~View.
   virtual void OnViewIsDeleting(View* observed_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_