[GTK] Remove GDK dependency from SharedTimerGtk
https://bugs.webkit.org/show_bug.cgi?id=138092

Reviewed by Martin Robinson.

It depends on GDK just to use GDK_PRIORITY_REDRAW constant, we can
use its value directly instead.

* PlatformGTK.cmake:
* platform/gtk/SharedTimerGtk.cpp:
(WebCore::setSharedTimerFireInterval):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@175245 268f45cc-cd09-0410-ab3c-d52691b4dbfc