See this topic on the CEF C++ forum for more details:
http://www.magpcss.org/ceforum/viewtopic.php?f=6&t=10641

Do the following changes in the CEF C++ sources:

1. In `chromium/src/cef/libcef/browser_impl_gtk.cc`:

    At the end of the CefBrowserImpl::UIT_CreateBrowser() function,
    before the return statement add the following code:

        gtk_widget_show_all(GTK_WIDGET(window_info_.m_Widget));

2. In `chromium/src/cef/libcef/webwidget_host_gtk.cc`:

    In WebWidgetHostGtkWidget::CreateNewWidget() function replace this line:

        gtk_box_pack_start(GTK_BOX(parent_view), widget, TRUE, TRUE, 0);

    With the following code:

        if (GTK_IS_BOX(parent_view)) {
          gtk_box_pack_start(GTK_BOX(parent_view), widget, TRUE, TRUE, 0);
        } else {
          // Parent view shouldn't contain any children, but in wxWidgets library
          // there will be GtkPizza widget for Panel or any other control.
          GList *children, *iter;
          children = gtk_container_get_children(GTK_CONTAINER(parent_view));
          GtkWidget* child = NULL;
          GtkWidget* vbox = gtk_vbox_new(FALSE, 0);
          for (iter = children; iter != NULL; iter = g_list_next(iter)) {
            child = GTK_WIDGET(iter->data);
            // We will have to keep a reference to that child that we remove,
            // otherwise we will get lots of warnings like "invalid unclassed
            // pointer in cast to `GtkPizza'". First we increase a reference,
            // we need to do this for a moment before we add this child to the
            // vbox, then we will decrease that reference.
            g_object_ref(G_OBJECT(child));
            gtk_container_remove(GTK_CONTAINER(parent_view), child);
          }
          g_list_free(children);
          gtk_box_pack_start(GTK_BOX(vbox), widget, TRUE, TRUE, 0);
          if (child != NULL) {
            // This child is packed to the box only so that its reference lives,
            // as it might be referenced from other code thus resulting in errors.
            gtk_box_pack_end(GTK_BOX(vbox), child, FALSE, FALSE, 0);
            gtk_widget_hide(GTK_WIDGET(child));
            g_object_unref(G_OBJECT(child));
          }
          gtk_widget_show(GTK_WIDGET(vbox));
          if (GTK_IS_SCROLLED_WINDOW(parent_view))
            gtk_scrolled_window_add_with_viewport(GTK_SCROLLED_WINDOW(parent_view), vbox);
          else
            gtk_container_add(GTK_CONTAINER(parent_view), vbox);
        }