DRWH: Set correct position for trainsient windows.

Transient surface expects position relative to the parent. Currently,
we pass the x and y coordinates of the transient window instead of
the relative points. This CL fixes the issue.
content_shell: Fix crash with menu.

We assume that a window without a parent is top level window and
set it as dispatcher. This is wrong in the case of content shell
as menu doesn't have a parent. Now, we check for the window type
before setting it as dispatcher.