views: Restore to old behaviour of Widget::ReparentNativeView().

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 5d1dfc445aee2d21e0b0e149ee891a1e8e6841d6..a455eddf26809e303c101406e72dd02b59b10ab7 100644
--- a/ui/views/widget/native_widget_aura.cc
+++ b/ui/views/widget/native_widget_aura.cc
@@ -1145,10 +1145,24 @@ void NativeWidgetPrivate::ReparentNativeView(gfx::NativeView native_view,
     (*it)->NotifyNativeViewHierarchyWillChange();
   }
 
-  if (new_parent)
+  if (new_parent) {
     new_parent->AddChild(native_view);
-  else if (previous_parent)
-    previous_parent->RemoveChild(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());
+  }
 
   // And now, notify them that they have a brand new parent.
   for (Widget::Widgets::iterator it = widgets.begin();