Use WmLayoutManager::OnChildWindowVisibilityChanged to update the system modal state.

ash/wm/system_modal_container_layout_manager_unittest.cc

Index: ash/wm/system_modal_container_layout_manager_unittest.cc
diff --git a/ash/wm/system_modal_container_layout_manager_unittest.cc b/ash/wm/system_modal_container_layout_manager_unittest.cc
index 9ba6dfac4d0118adbf62a2486161abe1e1f2a2e2..dc95fbdea4abf8c834b0b448bec58f1487410ba6 100644
--- a/ash/wm/system_modal_container_layout_manager_unittest.cc
+++ b/ash/wm/system_modal_container_layout_manager_unittest.cc
@@ -784,6 +784,16 @@ TEST_F(SystemModalContainerLayoutManagerTest, VisibilityChange) {
   EXPECT_TRUE(WmShell::Get()->IsSystemModalWindowOpen());
   EXPECT_TRUE(layout_manager->has_window_dimmer());
 
+  // Make sure that a child visibility change should not cause
+  // inconsistent state.
+  std::unique_ptr<aura::Window> child = base::MakeUnique<aura::Window>(nullptr);
+  child->SetType(ui::wm::WINDOW_TYPE_CONTROL);
+  child->Init(ui::LAYER_TEXTURED);
+  modal_window->AddChild(child.get());
+  child->Show();
+  EXPECT_TRUE(WmShell::Get()->IsSystemModalWindowOpen());
+  EXPECT_TRUE(layout_manager->has_window_dimmer());
+
   modal_window->Hide();
   EXPECT_FALSE(WmShell::Get()->IsSystemModalWindowOpen());
   EXPECT_FALSE(layout_manager->has_window_dimmer());