[GTK] correct tabstrip drag point calculation.

chrome/browser/gtk/tabs/dragged_tab_controller_gtk.cc

Index: chrome/browser/gtk/tabs/dragged_tab_controller_gtk.cc
diff --git a/chrome/browser/gtk/tabs/dragged_tab_controller_gtk.cc b/chrome/browser/gtk/tabs/dragged_tab_controller_gtk.cc
index 70ecdb8fcadf7c6708162a5e849b1b4ece2bb62c..194704b5d8773feb1916cf6e7202abc5655df6c3 100644
--- a/chrome/browser/gtk/tabs/dragged_tab_controller_gtk.cc
+++ b/chrome/browser/gtk/tabs/dragged_tab_controller_gtk.cc
@@ -438,9 +438,9 @@ void DraggedTabControllerGtk::Detach() {
 
 gfx::Point DraggedTabControllerGtk::ConvertScreenPointToTabStripPoint(
     TabStripGtk* tabstrip, const gfx::Point& screen_point) {
-  gint x, y;
-  gdk_window_get_origin(tabstrip->tabstrip_->window, &x, &y);
-  return gfx::Point(screen_point.x() - x, screen_point.y() - y);
+  gfx::Point tabstrip_screen_point =
+      gtk_util::GetWidgetScreenPosition(tabstrip->tabstrip_.get());
+  return screen_point.Subtract(tabstrip_screen_point);
 }
 
 gfx::Rect DraggedTabControllerGtk::GetDraggedTabTabStripBounds(