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 6fb1a08e84e7290ed73a1940d860b21bf3d1ca11..c84514babd39c3f70829665f2b8d6caf173360dc 100644
--- a/ash/display/cursor_window_controller.cc
+++ b/ash/display/cursor_window_controller.cc
@@ -120,9 +120,9 @@ 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);
+    AshWindowTreeHost* mirror_host = Shell::GetInstance()->
+        display_controller()->mirror_window_controller()->ash_host();
+    SetContainer(mirror_host ? mirror_host->AsWindowTreeHost()->window() : NULL);
   }
 }