views/mus: Add support for child widgets.

ui/views/widget/native_widget_aura.cc

Index: ui/views/widget/native_widget_aura.cc
diff --git a/ui/views/widget/native_widget_aura.cc b/ui/views/widget/native_widget_aura.cc
index 66022c0c6da18e1d9e493b653673dc18fdcb2c6b..5fb420fef944fdcddf6b007a588b6b2a8ba691fb 100644
--- a/ui/views/widget/native_widget_aura.cc
+++ b/ui/views/widget/native_widget_aura.cc
@@ -1143,24 +1143,10 @@ void NativeWidgetPrivate::ReparentNativeView(gfx::NativeView native_view,
     (*it)->NotifyNativeViewHierarchyWillChange();
   }
 
-  if (new_parent) {
+  if (new_parent)
     new_parent->AddChild(native_view);
-  } else {
-    // The following looks weird, but it's the equivalent of what aura has
-    // always done. (The previous behaviour of aura::Window::SetParent() used
-    // NULL as a special value that meant ask the WindowTreeClient where things
-    // should go.)
-    //
-    // This probably isn't strictly correct, but its an invariant that a Window
-    // in use will be attached to a RootWindow, so we can't just call
-    // RemoveChild here. The only possible thing that could assign a RootWindow
-    // in this case is the stacking client of the current RootWindow. This
-    // matches our previous behaviour; the global stacking client would almost
-    // always reattach the window to the same RootWindow.
-    aura::Window* root_window = native_view->GetRootWindow();
-    aura::client::ParentWindowWithContext(
-        native_view, root_window, root_window->GetBoundsInScreen());
-  }
+  else if (previous_parent)
+    previous_parent->RemoveChild(native_view);
 
   // And now, notify them that they have a brand new parent.
   for (Widget::Widgets::iterator it = widgets.begin();