Reviewed by Xan Lopez.

        [GTK] Port stock icon painting to GtkStyleContext
        https://bugs.webkit.org/show_bug.cgi?id=51764

        Port stock icon painting for media and search input elements to
        GtkStyleContext. Also create the initial machinery for accessing
        style contexts for all GTK+ 3.x based widgets.

        No new tests. This should not change functionality.

        * platform/gtk/RenderThemeGtk.cpp:
        (WebCore::paintGdkPixbuf):
        (WebCore::RenderThemeGtk::paintSearchFieldResultsDecoration):
        (WebCore::RenderThemeGtk::paintSearchFieldCancelButton):
        (WebCore::RenderThemeGtk::paintMediaButton):
        * platform/gtk/RenderThemeGtk.h:
        * platform/gtk/RenderThemeGtk2.cpp:
        (WebCore::RenderThemeGtk::getStockIcon):
        * platform/gtk/RenderThemeGtk3.cpp:
        (WebCore::gtkStyleChangedCallback):
        (WebCore::styleContextMap):
        (WebCore::getStyleContext):
        (WebCore::RenderThemeGtk::paintMenuList):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@74997 268f45cc-cd09-0410-ab3c-d52691b4dbfc