Revert of Refactor backdrop

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 d55c8b960a20d78edb65eaca5437a4905626f298..1fce7fc6b86075106060629c6377a76bf452bea3 100644
--- a/ash/wm/workspace_controller.h
+++ b/ash/wm/workspace_controller.h
@@ -15,7 +15,7 @@
 namespace ash {
 class WorkspaceEventHandler;
 class WorkspaceLayoutManager;
-class BackdropDelegate;
+class WorkspaceLayoutManagerBackdropDelegate;
 
 namespace test {
 class WorkspaceControllerTestApi;
@@ -37,7 +37,8 @@
 
   // Add a delegate which adds a backdrop behind the top window of the default
   // workspace.
-  void SetBackdropDelegate(std::unique_ptr<BackdropDelegate> delegate);
+  void SetMaximizeBackdropDelegate(
+      std::unique_ptr<WorkspaceLayoutManagerBackdropDelegate> delegate);
 
   WorkspaceLayoutManager* layout_manager() { return layout_manager_; }