Add child for parent_mus when params.parent_mus is not initialized.

ash/mus/test/wm_test_base.cc

Index: ash/mus/test/wm_test_base.cc
diff --git a/ash/mus/test/wm_test_base.cc b/ash/mus/test/wm_test_base.cc
index c45eb6e73457fd27f8e573754940c7f90f90f533..afee0f0b3376c5c5ccbe4eebec0ac07c3cd99d82 100644
--- a/ash/mus/test/wm_test_base.cc
+++ b/ash/mus/test/wm_test_base.cc
@@ -123,6 +123,65 @@ ui::Window* WmTestBase::CreateTestWindow(const gfx::Rect& bounds,
   return window;
 }
 
+RootWindowController* WmTestBase::GetPrimaryRootWindowController() {
+  std::vector<RootWindowController*> roots =
+      test_helper_->GetRootsOrderedByDisplayId();
+  DCHECK(!roots.empty());
+  return roots[0];
+}
+
+RootWindowController* WmTestBase::GetSecondaryRootWindowController() {
+  std::vector<RootWindowController*> roots =
+      test_helper_->GetRootsOrderedByDisplayId();
+  return roots.size() < 2 ? nullptr : roots[1];
+}
+
+ui::Window* WmTestBase::CreateTestWindow(const gfx::Rect& bounds,
+                                         ui::wm::WindowType window_type,
+                                         RootWindowController* root) {
+  std::map<std::string, std::vector<uint8_t>> properties;
+  properties[ui::mojom::WindowManager::kWindowType_Property] =
+      mojo::ConvertTo<std::vector<uint8_t>>(
+          static_cast<int32_t>(MusWindowTypeFromWmWindowType(window_type)));
+  if (!bounds.IsEmpty()) {
+    properties[ui::mojom::WindowManager::kInitialBounds_Property] =
+        mojo::ConvertTo<std::vector<uint8_t>>(bounds);
+  }
+  properties[ui::mojom::WindowManager::kResizeBehavior_Property] =
+      mojo::ConvertTo<std::vector<uint8_t>>(
+          ui::mojom::kResizeBehaviorCanResize |
+          ui::mojom::kResizeBehaviorCanMaximize |
+          ui::mojom::kResizeBehaviorCanMinimize);
+
+  properties[ui::mojom::WindowManager::kInitialDisplayId_Property] =
+      mojo::ConvertTo<std::vector<uint8_t>>(root->display().id());
+  // LOG(ERROR) << "\n  WmTestBase::CreateTestWindow
+  // properties[ui::mojom::WindowManager::kInitialDisplayId_Property]=" <<
+  // mojo::TypeConverter<int64_t,
+  // std::vector<uint8_t>>::Convert(properties[ui::mojom::WindowManager::kInitialDisplayId_Property]);
+
+  LOG(ERROR) << "\n root->display().id()" << root->display().id() << "\n";
+
+  ui::Window* window = root->window_manager()->NewTopLevelWindow(&properties);
+  window->SetVisible(true);
+  // Most tests expect a minimum size of 0x0.
+  WmWindowMusTestApi(WmWindowMus::Get(window)).set_use_empty_minimum_size(true);
+  return window;
+}
+
+ui::Window* WmTestBase::CreateFullscreenTestWindow(RootWindowController* root) {
+  std::map<std::string, std::vector<uint8_t>> properties;
+  properties[ui::mojom::WindowManager::kShowState_Property] =
+      mojo::ConvertTo<std::vector<uint8_t>>(
+          static_cast<int32_t>(ui::mojom::ShowState::FULLSCREEN));
+  properties[ui::mojom::WindowManager::kInitialDisplayId_Property] =
+      mojo::ConvertTo<std::vector<uint8_t>>(root->display().id());
+
+  ui::Window* window = root->window_manager()->NewTopLevelWindow(&properties);
+  window->SetVisible(true);
+  return window;
+}
+
 ui::Window* WmTestBase::CreateFullscreenTestWindow() {
   std::map<std::string, std::vector<uint8_t>> properties;
   properties[ui::mojom::WindowManager::kShowState_Property] =