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

ash/wm/workspace_controller.cc

Index: ash/wm/workspace_controller.cc
diff --git a/ash/wm/workspace_controller.cc b/ash/wm/workspace_controller.cc
index 55ccf64fd6b6e768d0fbf4e1cc50d34b5918c3f3..5fdcea4736e8acb951ac61e04c0503fd3a0d9004 100644
--- a/ash/wm/workspace_controller.cc
+++ b/ash/wm/workspace_controller.cc
@@ -135,5 +135,10 @@ void WorkspaceController::DoInitialAnimation() {
   }
 }
 
+void WorkspaceController::SetMaximizeBackdropDelegate(
+    MaximizeBackdropDelegate* delegate) {
+  layout_manager_->SetMaximizeBackdropDelegate(delegate);
+}
+
 }  // namespace internal
 }  // namespace ash