Reviewed by Adam. Manual test added.

        Fix for http://bugs.webkit.org/show_bug.cgi?id=12399 REGRESSION: 
        Unable to prevent default context menu from appearing. <rdar://
        problem/5017416>

        This problem appeared because of two facets of the current context 
        menu design. First, all context menu events are now considered to 
        be "swallowed" since we take care of building up the regular 
        context menu through the defaultEventHandler(). Second, the context 
        menu controller holds onto it's context menu until a new one is 
        created. There would be logistical problems changing this since 
        AppKit relies on the menu being around for as long as it is
        visible on the screen and we don't get any notification once the 
        menu is popped-down.

        This patch fixes the problem by giving WebKit a way to clear the 
        controller's context menu. WebKit now clears the menu before it 
        propagates a new context menu event through the DOM.

        * WebCore.exp:
        * manual-tests/DOMContextMenuEvent.html: Added.
        * page/ContextMenuController.cpp:
        (WebCore::ContextMenuController::clearContextMenu):
        * page/ContextMenuController.h:

WebKit:
        Reviewed by Adam.

        Fix for http://bugs.webkit.org/show_bug.cgi?id=12399 REGRESSION: 
        Unable to prevent default context menu from appearing. <rdar://
        problem/5017416>

        * WebView/WebHTMLView.mm:
        (-[WebHTMLView menuForEvent:]): Clear the controller's context menu 
        before propagating a new context menu event through the DOM.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@19810 268f45cc-cd09-0410-ab3c-d52691b4dbfc