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

ash/wm/workspace/workspace_layout_manager_delegate.h

Index: ash/wm/workspace/workspace_layout_manager_delegate.h
diff --git a/ash/wm/workspace/workspace_layout_manager_delegate.h b/ash/wm/workspace/workspace_layout_manager_delegate.h
new file mode 100644
index 0000000000000000000000000000000000000000..9d8934468dc9de8b3895e14776c151fd616910af
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+++ b/ash/wm/workspace/workspace_layout_manager_delegate.h
@@ -0,0 +1,49 @@
+// Copyright (c) 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef ASH_WM_WORKSPACE_WORKSPACE_LAYOUT_MANAGER_DELEGATE_H_
+#define ASH_WM_WORKSPACE_WORKSPACE_LAYOUT_MANAGER_DELEGATE_H_
+
+#include "ash/wm/wm_types.h"
+
+namespace aura {
+class Window;
+}
+
+namespace ash {
+namespace wm {
+class WindowState;
+}
+
+namespace internal {
+
+// A delegate which can be set to create and control a backdrop which gets
+// placed below the top level window.
+class WorkspaceLayoutManagerDelegate {
+ public:
+  WorkspaceLayoutManagerDelegate() {}
+  virtual ~WorkspaceLayoutManagerDelegate() {}
+
+  // A window got added to the layout.
+  virtual void OnWindowAddedToLayout(aura::Window* child) = 0;
+
+  // A window got removed from the layout.
+  virtual void OnWindowRemovedFromLayout(aura::Window* child) = 0;
+
+  // The visibility of a window has changed.
+  virtual void OnChildWindowVisibilityChanged(aura::Window* child,
+                                              bool visible) = 0;
+
+  // The stacking order of a window has changed.
+  virtual void OnWindowStackingChanged(aura::Window* window) = 0;
+
+  // A window state type has changed.
+  virtual void OnPostWindowStateTypeChange(wm::WindowState* window_state,
+                                           wm::WindowStateType old_type) = 0;
+};
+
+}  // namespace internal
+}  // namespace ash
+
+#endif  // ASH_WM_WORKSPACE_WORKSPACE_LAYOUT_MANAGER_DELEGATE_H_