Update Windows UI on system color changes.

ui/native_theme/native_theme.cc

 // 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.
 
 #include "ui/native_theme/native_theme.h"
 
 namespace ui {
 
+NativeThemeObserver::~NativeThemeObserver() {}
+
 void NativeTheme::SetScrollbarColors(unsigned inactive_color,
                                      unsigned active_color,
                                      unsigned track_color) {
   thumb_inactive_color_ = inactive_color;
   thumb_active_color_ = active_color;
   track_color_ = track_color;
 }
 
 // NativeTheme::instance() is implemented in the platform specific source files,
 // such as native_theme_win.cc or native_theme_linux.cc
 
+void NativeTheme::AddObserver(NativeThemeObserver* observer) {
+  native_theme_observers_.AddObserver(observer);
+}
+
+void NativeTheme::RemoveObserver(NativeThemeObserver* observer) {
+  native_theme_observers_.RemoveObserver(observer);
+}
+
+void NativeTheme::OnNativeThemeChange() {
+  FOR_EACH_OBSERVER(NativeThemeObserver, native_theme_observers_,
+                    OnNativeThemeChange());
+}
+
 NativeTheme::NativeTheme()
     : thumb_inactive_color_(0xeaeaea),
       thumb_active_color_(0xf4f4f4),
-      track_color_(0xd3d3d3) {
+      track_color_(0xd3d3d3),
+      color_change_listener_(this) {
 }
 
 NativeTheme::~NativeTheme() {}
 
+void NativeTheme::OnSysColorChange() {
+  OnNativeThemeChange();

[Evan Stade, 2014/04/22 17:43:44]

The native theme won't always change just because the system colors have changed
-- it depends on the theme. I don't think the base NativeTheme class should do
this; leave it up to subclasses.

[msw, 2014/04/22 19:48:58]

Sadly, I'd have to do this for NativeThemeWin and NativeThemeAuraWin (which uses
NativeThemeAura as its base class) if not here. Also, SysColorChangeListener
could possibly be extended in a followup CL to catch the GTK frame "style-set"
signal and then this would be applicable for NativeThemeGtk2. Thoughts?

[Evan Stade, 2014/04/22 21:09:18]

I don't mind that you'd have to do this in more than one place; that's expected.
The amount of duplicated code is minimal.

[msw, 2014/04/23 00:34:04]

I moved this over to NativeThemeWin; the observation by NativeThemeAuraWin is no
longer needed in the latest patch set, where Widget is the NativeThemeObserver
and correctly observes only NativeThemeWin.

+}
+
 }  // namespace ui