GDK.gdk_window_get_frame_extents (window, rect).

For GDK.gdk_window_get_frame_extents (window, rect) to return correct
values shell should be open. Once SWT asks GTK to show the shell via
Native calls it takes sometime to open the shell.
On high performance machines adjustTrip is asking GTK to give it width
and height before it has completely opened the shell. This results in
returning default size.(which is {0,0,1,1})
So We can use the height and width which we already know when opening
the shell.

 Fixes https://github.com/eclipse-platform/eclipse.platform.swt/issues/1161