<rdar://problem/5395928> Need to be able to handle context menu item selectio...

    <rdar://problem/5395928> Need to be able to handle context menu item selection by index

        Reviewed by Beth.

WebCore:
        * platform/ContextMenu.h: Added method to retrieve context menu item by index
        * platform/win/ContextMenuWin.cpp:
        (WebCore::ContextMenu::ContextMenu): call setPlatformDescription() since that will handle
        adding the MNS_NOTIFYBYPOS style to the context menu.
        (WebCore::contextMenuItemByIdOrPosition): helper method so we don't have to duplicate code
        between itemWithAction() and itemAtIndex().
        (WebCore::ContextMenu::itemWithAction):
        (WebCore::ContextMenu::itemAtIndex):
        (WebCore::ContextMenu::setPlatformDescription): add MNS_NOTIFYBYPOS style to the context menu
        so we will get notified by menu position through WM_MENUCOMMAND when the item is selected.

WebKit/win:
        * WebView.cpp:
        (WebView::performContextMenuAction): performContextMenuAction() can now handle context menu
        item selection via WM_MENUCOMMAND (by specifying byPosition to be true).  In that case, we
        get the ContextMenuItem by position rather than by action id.
        (WebViewWndProc): handle WM_MENUCOMMAND
        * WebView.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@25426 268f45cc-cd09-0410-ab3c-d52691b4dbfc