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..8d159b7d0c8e71946ce5ba6090f1935b8d9b3bc5 100644
--- a/ash/wm/workspace_controller.h
+++ b/ash/wm/workspace_controller.h
@@ -37,6 +37,9 @@ 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 AddBackdropBehindTopWindow(bool enable);
+
  private:
   friend class WorkspaceControllerTestHelper;