Reviewed by Martin Robinson.

        [GTK] Implement spin buttons in RenderThemeGtk
        https://bugs.webkit.org/show_bug.cgi?id=51454

        Paint inner up/down buttons for spin button elements when building
        with GTK+ 3.x.

        Test results will land with the GTK+ 2.x version of this patch.

        * platform/gtk/RenderThemeGtk.h:
        * platform/gtk/RenderThemeGtk2.cpp:
        (WebCore::RenderThemeGtk::adjustInnerSpinButtonStyle):
        (WebCore::RenderThemeGtk::paintInnerSpinButton):
        * platform/gtk/RenderThemeGtk3.cpp:
        (WebCore::spinButtonArrowSize):
        (WebCore::RenderThemeGtk::adjustInnerSpinButtonStyle):
        (WebCore::paintSpinArrowButton):
        (WebCore::RenderThemeGtk::paintInnerSpinButton):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76577 268f45cc-cd09-0410-ab3c-d52691b4dbfc