Introdcue AshWindowTreeHost and move ash/chrome specific code in WTH to ash.

ash/desktop_background/desktop_background_controller.cc

Index: ash/desktop_background/desktop_background_controller.cc
diff --git a/ash/desktop_background/desktop_background_controller.cc b/ash/desktop_background/desktop_background_controller.cc
index 730558696d93d2ce82e7a7dcb78b172c32d568d2..2a039e97230760f07c4b84c176405ac586787ac4 100644
--- a/ash/desktop_background/desktop_background_controller.cc
+++ b/ash/desktop_background/desktop_background_controller.cc
@@ -411,10 +411,10 @@ bool DesktopBackgroundController::ReparentBackgroundWidgets(int src_container,
     DesktopBackgroundWidgetController* desktop_controller =
         root_window_controller->wallpaper_controller();
     if (desktop_controller) {
-      moved |= desktop_controller->Reparent(
-          root_window_controller->root_window(),
-          src_container,
-          dst_container);
+      moved |=
+          desktop_controller->Reparent(root_window_controller->GetRootWindow(),
+                                       src_container,
+                                       dst_container);
     }
     // During desktop show animations the controller lives in
     // AnimatingDesktopController owned by RootWindowController.
@@ -428,7 +428,7 @@ bool DesktopBackgroundController::ReparentBackgroundWidgets(int src_container,
         NULL;
     if (animating_controller) {
       moved |= animating_controller->Reparent(
-          root_window_controller->root_window(),
+          root_window_controller->GetRootWindow(),
           src_container,
           dst_container);
     }