Update Windows UI on system color changes.

ui/native_theme/native_theme.h

Index: ui/native_theme/native_theme.h
diff --git a/ui/native_theme/native_theme.h b/ui/native_theme/native_theme.h
index 67af252d7bd849273056b306ab233ceddf78bcbb..2554586b692a7ead4edae6f1627ad912396639da 100644
--- a/ui/native_theme/native_theme.h
+++ b/ui/native_theme/native_theme.h
@@ -5,6 +5,7 @@
 #ifndef UI_NATIVE_THEME_NATIVE_THEME_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_H_
 
+#include "base/observer_list.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "ui/gfx/native_widget_types.h"
 #include "ui/native_theme/native_theme_export.h"
@@ -18,6 +19,15 @@ class Size;
 
 namespace ui {
 
+// Observers which are notified when the native theme changes.
+class NATIVE_THEME_EXPORT NativeThemeObserver {
+ public:
+  virtual ~NativeThemeObserver();
+
+  // Called whenever the underlying platform's native theme changes.
+  virtual void OnNativeThemeUpdate() = 0;
+};
+
 // This class supports drawing UI controls (like buttons, text fields, lists,
 // comboboxes, etc) that look like the native UI controls of the underlying
 // platform, such as Windows or Linux. It also supplies default colors for
@@ -310,7 +320,14 @@ class NATIVE_THEME_EXPORT NativeTheme {
   // function, returning the port's subclass.
   static NativeTheme* instance();
 
+  // Add or remove observers to be notified when the native theme changes.
+  void AddObserver(NativeThemeObserver* observer);
+  void RemoveObserver(NativeThemeObserver* observer);
+
  protected:
+  // Notify observers of native theme state changes.
+  void NotifyObservers();
+
   NativeTheme();
   virtual ~NativeTheme();
 
@@ -318,6 +335,10 @@ class NATIVE_THEME_EXPORT NativeTheme {
   unsigned int thumb_active_color_;
   unsigned int track_color_;
 
+ private:
+  // Observers to notify when the theme state changes.
+  ObserverList<NativeThemeObserver> native_theme_observers_;
+
   DISALLOW_COPY_AND_ASSIGN(NativeTheme);
 };