frames: Explicitly initialize style contexts

We were relying on GTK+ emitting GtkWidget::style-updated during
widget initialization to create the GtkStyleContexts used for
window decorations. A recent GTK+ update broke this assumption,
so do the necessary initialization ourselves.

https://bugzilla.gnome.org/show_bug.cgi?id=671796

src/ui/frames.c

@@ -273,6 +273,7 @@ meta_frames_init (MetaFrames *frames)
 
   frames->style_variants = g_hash_table_new_full (g_str_hash, g_str_equal,
                                                   g_free, g_object_unref);
+  update_style_contexts (frames);
 
   gtk_widget_set_double_buffered (GTK_WIDGET (frames), FALSE);