Adding a gray semi transparent backdrop behind the topmost window within the default container

ash/wm/workspace/workspace_layout_manager.h

Index: ash/wm/workspace/workspace_layout_manager.h
diff --git a/ash/wm/workspace/workspace_layout_manager.h b/ash/wm/workspace/workspace_layout_manager.h
index 1c20d2edafcefbfe0577450426088eeaded03544..ff15728179044abb3d17eda115cdd954af646b2c 100644
--- a/ash/wm/workspace/workspace_layout_manager.h
+++ b/ash/wm/workspace/workspace_layout_manager.h
@@ -13,6 +13,7 @@
 #include "ash/wm/wm_types.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
+#include "base/memory/scoped_ptr.h"
 #include "ui/aura/client/activation_change_observer.h"
 #include "ui/aura/layout_manager.h"
 #include "ui/aura/window_observer.h"
@@ -36,6 +37,7 @@ class WMEvent;
 namespace internal {
 
 class ShelfLayoutManager;
+class WorkspaceLayoutManagerDelegate;
 
 // LayoutManager used on the window created for a workspace.
 class ASH_EXPORT WorkspaceLayoutManager
@@ -50,6 +52,12 @@ class ASH_EXPORT WorkspaceLayoutManager
 
   void SetShelf(internal::ShelfLayoutManager* shelf);
 
+  // A delegate which can be set to add a backdrop behind the top most visible
+  // window. With the call the ownership of the delegate will be transferred to
+  // the WorkspaceLayoutManager.
+  void SetMaximizeBackdropDelegate(
+      scoped_ptr<WorkspaceLayoutManagerDelegate> delegate);
+
   // Overridden from aura::LayoutManager:
   virtual void OnWindowResized() OVERRIDE {}
   virtual void OnWindowAddedToLayout(aura::Window* child) OVERRIDE;
@@ -128,6 +136,10 @@ class ASH_EXPORT WorkspaceLayoutManager
   // True if this workspace is currently in fullscreen mode.
   bool is_fullscreen_;
 
+  // A window which covers the full container and which gets inserted behind the
+  // topmost visible window.
+  scoped_ptr<WorkspaceLayoutManagerDelegate> backdrop_delegate_;
+
   DISALLOW_COPY_AND_ASSIGN(WorkspaceLayoutManager);
 };