Fix white flash when user sings out and wallpaper animation regression

ash/desktop_background/desktop_background_widget_controller.h

Index: ash/desktop_background/desktop_background_widget_controller.h
diff --git a/ash/desktop_background/desktop_background_widget_controller.h b/ash/desktop_background/desktop_background_widget_controller.h
index fe07ef30430b656ed0ad7f90537e09f5de62b491..c50c4fa580c21507b90c96b9ea4213efd2b4502f 100644
--- a/ash/desktop_background/desktop_background_widget_controller.h
+++ b/ash/desktop_background/desktop_background_widget_controller.h
@@ -15,6 +15,11 @@ namespace internal {
 
 // This class hides difference between two possible background implementations:
 // effective Layer-based for solid color, and Widget-based for images.
+// DesktopBackgroundWidgetController is installed as an owned property on the
+// RootWindow. To avoid a white flash during wallpaper changes the old
+// DesktopBackgroundWidgetController is moved to a secondary property
+// (kComponentWrapper). When the animation completes the old
+// DesktopBackgroundWidgetController is destroyed.
 class DesktopBackgroundWidgetController {
  public:
   // Create
@@ -43,11 +48,31 @@ class DesktopBackgroundWidgetController {
   DISALLOW_COPY_AND_ASSIGN(DesktopBackgroundWidgetController);
 };
 
+// This class wraps a DesktopBackgroundWidgetController pointer. It is installed
+// as an owned property on the RootWindow. DesktopBackgroundWidgetController is
+// moved to this property before animation completes. After animation completes,
+// the kWindowDesktopComponent property on RootWindow is set to the
+// DesktopBackgroundWidgetController in this class.
+class ComponentWrapper {
+ public:
+  explicit ComponentWrapper(
+      DesktopBackgroundWidgetController* component);
+  ~ComponentWrapper() {}
+  DesktopBackgroundWidgetController* component() { return component_; }
+
+ private:
+  DesktopBackgroundWidgetController* component_;
+
+  DISALLOW_COPY_AND_ASSIGN(ComponentWrapper);
+};
+
 // Window property key, that binds instance of DesktopBackgroundWidgetController
 // to root windows.
 extern const aura::WindowProperty<DesktopBackgroundWidgetController*>* const
     kWindowDesktopComponent;
 
+extern const aura::WindowProperty<ComponentWrapper*>* const kComponentWrapper;
+
 }  // namespace internal
 }  // namespace ash