[GTK] Add a helper function for display system deduction
https://bugs.webkit.org/show_bug.cgi?id=136849

Reviewed by Martin Robinson.

getDisplaySystemType() is a helper function that checks the type of the
default GdkDisplay object and initializes a static variable to the
corresponding value. It then returns the value of that variable.

When building with the GTK+2 API (still the case with the GTK+2 plugin
process) we default to X11.

The function is declared and implemented in GtkUtilities files.

* platform/gtk/GtkUtilities.cpp:
(WebCore::widgetIsOnscreenToplevelWindow):
(WebCore::getDisplaySystemType):
* platform/gtk/GtkUtilities.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@173690 268f45cc-cd09-0410-ab3c-d52691b4dbfc