coverity#1209550 Unchecked dynamic_cast

Dynamic cast can be dangerous - if the cast fails then it returns NULL.
Coverity picked this up - it could effect
Window::IsNativeWidgetEndabled(). Hopefully I'll get rid of this when I
remove meOutDevType during work on fdo#74702.

Change-Id: Id6df1eeff716a6acd1b108b5e1e4674e819afe3d