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

ui/views/mus/native_widget_mus.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This has to be before any other includes, else default is picked up.
 // See base/logging for details on this.
 #define NOTIMPLEMENTED_POLICY 5
 
 #include "ui/views/mus/native_widget_mus.h"
 
@@ skipping 757 matching lines @@
 
   content_->SetType(ui::wm::WINDOW_TYPE_NORMAL);
   content_->Init(params.layer_type);
   if (window_->visible())
     content_->Show();
   content_->SetTransparent(true);
   content_->SetFillsBoundsCompletely(false);
   content_->set_ignore_events(!params.accept_events);
   hosted_window->AddChild(content_);
 
+  ui::Window* parent_mus = params.parent_mus;
+
   // Set-up transiency if appropriate.
   if (params.parent && !params.child) {
-    aura::Window* parent_root = params.parent->GetRootWindow();
-    ui::Window* parent_mus = parent_root->GetProperty(kMusWindow);
-    if (parent_mus)
-      parent_mus->AddTransientWindow(window_);
+    aura::Window* parent_root_aura = params.parent->GetRootWindow();
+    ui::Window* parent_root_mus = parent_root_aura->GetProperty(kMusWindow);
+    if (parent_root_mus) {
+      parent_root_mus->AddTransientWindow(window_);
+      if (!parent_mus)
+        parent_mus = parent_root_mus;
+    }
   }
 
-  if (params.parent_mus)
-    params.parent_mus->AddChild(window_);
+  if (parent_mus)
+    parent_mus->AddChild(window_);
 
   // TODO(sky): deal with show state.
   if (!params.bounds.size().IsEmpty())
     SetBounds(params.bounds);
 
   // TODO(beng): much else, see [Desktop]NativeWidgetAura.
 
   native_widget_delegate_->OnNativeWidgetCreated(false);
 }
 
@@ skipping 771 matching lines @@
 
   gfx::Path mask_path;
   native_widget_delegate_->GetHitTestMask(&mask_path);
   // TODO(jamescook): Use the full path for the mask.
   gfx::Rect mask_rect =
       gfx::ToEnclosingRect(gfx::SkRectToRectF(mask_path.getBounds()));
   window_->SetHitTestMask(mask_rect);
 }
 
 }  // namespace views