[GTK] Menulist buttons have separators even when the theme turns them off

[GTK] Menulist buttons have separators even when the theme turns them off
https://bugs.webkit.org/show_bug.cgi?id=80668

Reviewed by Daniel Bates.

No new tests. GTK+ theme differences are notoriously difficult
to test, because consistent results depend on having certain themes
and certain versions of themes installed.

Instead of using the GTK_TYPE_BUTTON and GTK_TYPE_SEPARATOR tags to get the
style context, use GTK_TYPE_COMBO_BOX which should provide more accurate theme settings.

* platform/gtk/RenderThemeGtk3.cpp:
(WebCore::getComboBoxMetrics): Get metrics from a GTK_TYPE_COMBO_BOX style context.
(WebCore::RenderThemeGtk::paintMenuList): Get separator settings from the GTK_TYPE_COMBO_BOX style context.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@110762 268f45cc-cd09-0410-ab3c-d52691b4dbfc