Refactors DimWindow and moves to ash/common

ash/wm/screen_pinning_controller.cc

Index: ash/wm/screen_pinning_controller.cc
diff --git a/ash/wm/screen_pinning_controller.cc b/ash/wm/screen_pinning_controller.cc
index a74dffe05f93f21f96de24006be5330650e9e033..f94bb826f12afadc97cee5ba143f25c7594a38ea 100644
--- a/ash/wm/screen_pinning_controller.cc
+++ b/ash/wm/screen_pinning_controller.cc
@@ -9,13 +9,13 @@
 
 #include "ash/aura/wm_window_aura.h"
 #include "ash/common/shell_window_ids.h"
+#include "ash/common/wm/window_dimmer.h"
 #include "ash/common/wm/window_state.h"
 #include "ash/common/wm_shell.h"
 #include "ash/common/wm_window.h"
 #include "ash/common/wm_window_observer.h"
 #include "ash/display/window_tree_host_manager.h"
 #include "ash/shell.h"
-#include "ash/wm/dim_window.h"
 #include "base/auto_reset.h"
 #include "base/logging.h"
 #include "base/stl_util.h"
@@ -25,12 +25,12 @@ namespace ash {
 namespace {
 
 WmWindow* CreateDimWindow(WmWindow* container) {
-  DimWindow* dim_window = new DimWindow(WmWindowAura::GetAuraWindow(container));
-  dim_window->SetDimOpacity(1);  // Set whole black.
-  WmWindow* result = WmWindowAura::Get(dim_window);
-  result->SetFullscreen();
-  result->Show();
-  return result;
+  // WindowDimmer is owned by |container|.
+  WindowDimmer* window_dimmer = new WindowDimmer(container);
+  window_dimmer->SetDimOpacity(1);  // Fully opaque.
+  window_dimmer->window()->SetFullscreen();
+  window_dimmer->window()->Show();
+  return window_dimmer->window();
 }
 
 // Returns a list of WmWindows corresponding to SystemModalContainers,