Creating a WebInspector.ContextMenu without an event crashes WebCore when calling .show()
https://bugs.webkit.org/show_bug.cgi?id=108636

Reviewed by Pavel Feldman.

Return early if InspectorFrontendHost::showContextMenu() is called without an event and
assert in ContextMenuController::createContextMenu() in case no event was provided.

* inspector/InspectorFrontendHost.cpp:
(WebCore::InspectorFrontendHost::showContextMenu):
* page/ContextMenuController.cpp:
(WebCore::ContextMenuController::createContextMenu):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141692 268f45cc-cd09-0410-ab3c-d52691b4dbfc