[GTK] Scrollbars incorrectly rendered with older versions of GTK+

https://bugs.webkit.org/show_bug.cgi?id=153861

Reviewed by Michael Catanzaro.

The theme doesn't really know it's a scrollbar. Older versions of
GTK+ require to explicitly add the scrollbar style class to the
child GtkStyleContext.

* platform/gtk/ScrollbarThemeGtk.cpp:
(WebCore::createChildStyleContext):

Canonical link: https://commits.webkit.org/172010@main
git-svn-id: https://svn.webkit.org/repository/webkit/trunk@196172 268f45cc-cd09-0410-ab3c-d52691b4dbfc

Source/WebCore/ChangeLog

@@ -1,3 +1,17 @@
+2016-02-05  Carlos Garcia Campos  <cgarcia@igalia.com>
+
+        [GTK] Scrollbars incorrectly rendered with older versions of GTK+
+        https://bugs.webkit.org/show_bug.cgi?id=153861
+
+        Reviewed by Michael Catanzaro.
+
+        The theme doesn't really know it's a scrollbar. Older versions of
+        GTK+ require to explicitly add the scrollbar style class to the
+        child GtkStyleContext.
+
+        * platform/gtk/ScrollbarThemeGtk.cpp:
+        (WebCore::createChildStyleContext):
+
 2016-02-05  Carlos Garcia Campos  <cgarcia@igalia.com>
 
         [GTK] Scrollbars not correctly rendered in non GNOME environments

Source/WebCore/platform/gtk/ScrollbarThemeGtk.cpp

@@ -220,6 +220,7 @@ static GRefPtr<GtkStyleContext> createChildStyleContext(GtkStyleContext* parent,
 #if GTK_CHECK_VERSION(3, 19, 2)
     gtk_widget_path_iter_set_object_name(path.get(), -1, name);
 #else
+    gtk_widget_path_iter_add_class(path.get(), -1, GTK_STYLE_CLASS_SCROLLBAR);
     gtk_widget_path_iter_add_class(path.get(), -1, name);
 #endif