Refactor backdrop that is currently used in the maximized mode.

ash/test/workspace_controller_test_api.cc

Index: ash/test/workspace_controller_test_api.cc
diff --git a/ash/test/workspace_controller_test_api.cc b/ash/test/workspace_controller_test_api.cc
index ffd6e7c5b8737e039063586de115f378e5be1b66..ecc54aff0564651c337f77b86124af141e9005f8 100644
--- a/ash/test/workspace_controller_test_api.cc
+++ b/ash/test/workspace_controller_test_api.cc
@@ -5,6 +5,8 @@
 #include "ash/test/workspace_controller_test_api.h"
 
 #include "ash/test/workspace_event_handler_test_helper.h"
+#include "ash/wm/workspace/backdrop_controller.h"
+#include "ash/wm/workspace/workspace_layout_manager.h"
 #include "ash/wm/workspace_controller.h"
 #include "ui/aura/window.h"
 
@@ -26,5 +28,9 @@ WorkspaceControllerTestApi::GetMultiWindowResizeController() {
   return WorkspaceEventHandlerTestHelper(GetEventHandler()).resize_controller();
 }
 
+aura::Window* WorkspaceControllerTestApi::GetBackdropWindow() {
+  return controller_->layout_manager_->backdrop_controller_->backdrop_window_;
+}
+
 }  // namespace test
 }  // namespace ash