Reviewed by Alice.

    Reviewed by Alice.

        - Fix for <rdar://problem/4707372>
          After timers fix, crash in -[NSMenu performActionForItemAtIndex:] after moving/detroying active popup menu

        * platform/PopupMenu.h: (WebCore::PopupMenu::menuListIsDetaching): Added. Zeros out the pointer to the menu list.
        * rendering/RenderMenuList.cpp: (WebCore::RenderMenuList::~RenderMenuList): Calls menuListIsDetaching.
        * platform/mac/PopupMenuMac.mm: (WebCore::PopupMenu::show): Added a RefPtr to protect the PopupMenu while the NSMenu is up,
          in case the RenderMenuList goes away.  Also, checks the pointer to the menu list immediately afterwards.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@17022 268f45cc-cd09-0410-ab3c-d52691b4dbfc