Refactors DimWindow and moves to ash/common

ash/wm/screen_dimmer_unittest.cc

Index: ash/wm/screen_dimmer_unittest.cc
diff --git a/ash/wm/screen_dimmer_unittest.cc b/ash/wm/screen_dimmer_unittest.cc
index e94c727ac29e1a0e0f6974215aff27830a4e70cc..c00a5dab5618fe614d3ce22e399614cc7a8a5aa1 100644
--- a/ash/wm/screen_dimmer_unittest.cc
+++ b/ash/wm/screen_dimmer_unittest.cc
@@ -6,11 +6,11 @@
 
 #include <memory>
 
+#include "ash/aura/wm_window_aura.h"
+#include "ash/common/wm/window_dimmer.h"
 #include "ash/root_window_controller.h"
 #include "ash/shell.h"
 #include "ash/test/ash_test_base.h"
-#include "ash/wm/dim_window.h"
-//#include "ui/aura/window_event_dispatcher.h"
 #include "ui/aura/test/test_windows.h"
 #include "ui/compositor/layer.h"
 
@@ -28,7 +28,10 @@ class ScreenDimmerTest : public AshTestBase {
   }
 
   aura::Window* GetDimWindow() {
-    return DimWindow::Get(Shell::GetPrimaryRootWindow());
+    WindowDimmer* window_dimmer =
+        WindowDimmer::Get(WmWindowAura::Get(Shell::GetPrimaryRootWindow()));
+    return window_dimmer ? WmWindowAura::GetAuraWindow(window_dimmer->window())
+                         : nullptr;
   }
 
   ui::Layer* GetDimWindowLayer() {