Refactors DimWindow and moves to ash/common

ash/wm/system_modal_container_layout_manager.h

Index: ash/wm/system_modal_container_layout_manager.h
diff --git a/ash/wm/system_modal_container_layout_manager.h b/ash/wm/system_modal_container_layout_manager.h
index 9f20e24249333e40aa8fd6777986e8f3fd70ab54..bdfbb061f35d4a2958193d9032a0c1d9481d14a3 100644
--- a/ash/wm/system_modal_container_layout_manager.h
+++ b/ash/wm/system_modal_container_layout_manager.h
@@ -23,7 +23,7 @@ class Rect;
 }
 
 namespace ash {
-class DimWindow;
+class WindowDimmer;
 
 // LayoutManager for the modal window container.
 // System modal windows which are centered on the screen will be kept centered
@@ -36,7 +36,7 @@ class ASH_EXPORT SystemModalContainerLayoutManager
   explicit SystemModalContainerLayoutManager(aura::Window* container);
   ~SystemModalContainerLayoutManager() override;
 
-  bool has_modal_background() const { return modal_background_ != NULL; }
+  bool has_window_dimmer() const { return window_dimmer_ != nullptr; }
 
   // Overridden from SnapToPixelLayoutManager:
   void OnWindowResized() override;
@@ -97,9 +97,9 @@ class ASH_EXPORT SystemModalContainerLayoutManager
   // The container that owns the layout manager.
   aura::Window* container_;
 
-  // A window that dims the windows behind the modal window(s) being
-  // shown in |container_|.
-  DimWindow* modal_background_;
+  // WindowDimmer used to dim windows behind the modal window(s) being shown in
+  // |container_|.
+  WindowDimmer* window_dimmer_ = nullptr;
 
   // A stack of modal windows. Only the topmost can receive events.
   std::vector<aura::Window*> modal_windows_;