[GTK] Don't show accel labels in context menu items

[GTK] Don't show accel labels in context menu items
https://bugs.webkit.org/show_bug.cgi?id=90437

Reviewed by Martin Robinson.

When a context menu item is created with a GtkMenuItem that has
been created from a GtkAction, if the action has an accelerator,
it's shown in the menu item label, like menu items in a menu
bar. In that case we should reset the accel closure of the menu
item label, like GtkUIManager does for popup menus, to make sure
the accelerator is not shown. This isn't needed for all other
ContextMenuIem constructors, because the GtkAction is created
without accelerator by webkit in those cases.

* platform/gtk/ContextMenuItemGtk.cpp:
(WebCore::ContextMenuItem::ContextMenuItem):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121829 268f45cc-cd09-0410-ab3c-d52691b4dbfc