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

ash/wm/workspace_controller.h

Index: ash/wm/workspace_controller.h
diff --git a/ash/wm/workspace_controller.h b/ash/wm/workspace_controller.h
index 84df41c91d3c1d07ee193241399e4282280823ae..1b7c1a83f9036ec9ccd18d5f5264fef4529c7674 100644
--- a/ash/wm/workspace_controller.h
+++ b/ash/wm/workspace_controller.h
@@ -21,6 +21,7 @@ class ShelfLayoutManager;
 class WorkspaceControllerTestHelper;
 class WorkspaceEventHandler;
 class WorkspaceLayoutManager;
+class MaximizeBackdropDelegate;
 
 // WorkspaceController acts as a central place that ties together all the
 // various workspace pieces.
@@ -37,6 +38,9 @@ class ASH_EXPORT WorkspaceController {
   // Starts the animation that occurs on first login.
   void DoInitialAnimation();
 
+  // Add a backdrop behind the top window of the default workspace.
+  void SetMaximizeBackdropDelegate(MaximizeBackdropDelegate* enable);
+
  private:
   friend class WorkspaceControllerTestHelper;