Adds OnChildWindowVisibilityChanged to ui::WindowObserver

ash/mus/workspace/workspace_layout_manager_unittest.cc

Index: ash/mus/workspace/workspace_layout_manager_unittest.cc
diff --git a/ash/mus/workspace/workspace_layout_manager_unittest.cc b/ash/mus/workspace/workspace_layout_manager_unittest.cc
index 9f58d2e050c36df01118efefdabcc13e6d55d81e..f5e1799e6e63602a730a43962325e4a24306dd18 100644
--- a/ash/mus/workspace/workspace_layout_manager_unittest.cc
+++ b/ash/mus/workspace/workspace_layout_manager_unittest.cc
@@ -524,8 +524,8 @@ class FocusObserver : public ui::WindowObserver {
   ~FocusObserver() override { window_->RemoveObserver(this); }
 
   // aura::test::TestWindowDelegate overrides:
-  void OnWindowVisibilityChanged(ui::Window* window) override {
-    if (window_->visible())
+  void OnWindowVisibilityChanged(ui::Window* window, bool visible) override {
+    if (visible)
       window_->SetFocus();
     show_state_ = WmWindowMus::Get(window_)->GetShowState();
   }