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 ffaa10a88fc94fe259fcaab50f6ce2f707209334..e24053cea2d0107a9c26933efb09c8a519cade59 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"
@@ -35,6 +36,7 @@ class WindowState;
 namespace internal {
 
 class ShelfLayoutManager;
+class WorkspaceLayoutManagerBackdrop;
 
 // LayoutManager used on the window created for a workspace.
 class ASH_EXPORT WorkspaceLayoutManager
@@ -49,6 +51,10 @@ class ASH_EXPORT WorkspaceLayoutManager
 
   void SetShelf(internal::ShelfLayoutManager* shelf);
 
+  // If enabled, the topmost window in the container will have a backdrop which
+  // covers the rest of the container.
+  void AddBackdropBehindTopWindow(bool enable);
+
   // Overridden from aura::LayoutManager:
   virtual void OnWindowResized() OVERRIDE {}
   virtual void OnWindowAddedToLayout(aura::Window* child) OVERRIDE;
@@ -95,6 +101,9 @@ class ASH_EXPORT WorkspaceLayoutManager
   // Updates the visibility state of the shelf.
   void UpdateShelfVisibility();
 
+  // Updates the visibility of the backdrop.
+  void UpdateStackingAndVisibilityOfBackdrop();
+
   // Updates the fullscreen state of the workspace and notifies Shell if it
   // has changed.
   void UpdateFullscreenState();
@@ -124,6 +133,10 @@ 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<WorkspaceLayoutManagerBackdrop> backdrop_;
+
   DISALLOW_COPY_AND_ASSIGN(WorkspaceLayoutManager);
 };