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

ash/display/cursor_window_controller.cc

Index: ash/display/cursor_window_controller.cc
diff --git a/ash/display/cursor_window_controller.cc b/ash/display/cursor_window_controller.cc
index d79f5c55eaf4ca329c5e51b4d78265e4c9841c1e..ad3e9d18a5337d07e5d2efe039002c0454ffcd45 100644
--- a/ash/display/cursor_window_controller.cc
+++ b/ash/display/cursor_window_controller.cc
@@ -117,9 +117,10 @@ void CursorWindowController::UpdateContainer() {
   if (is_cursor_compositing_enabled_) {
     SetDisplay(display_);
   } else {
-    aura::WindowTreeHost* mirror_host = Shell::GetInstance()->
-        display_controller()->mirror_window_controller()->host();
-    SetContainer(mirror_host ? mirror_host->window() : NULL);
+    SetContainer(Shell::GetInstance()->
+                 display_controller()->
+                 mirror_window_controller()->
+                 GetWindow());
   }
 }