RenderThemeGtk depends on classes outside of platform
https://bugs.webkit.org/show_bug.cgi?id=22176

Reviewed by Martin Robinson.

Move RenderThemeGtk.cpp from platform to rendering.

* PlatformGTK.cmake:
* rendering/RenderThemeGtk.cpp: Renamed from Source/WebCore/platform/gtk/RenderThemeGtk.cpp.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@173111 268f45cc-cd09-0410-ab3c-d52691b4dbfc