Test that when window.onerror handler is called, window.event is the corresponding ErrorEvent object. Bug 8519.

Main frame window.onerror: Error: 2010 at window-onerror9.html, line: 32, column: 22
window.event.type = error
window.event.message = Error: 2010
window.event.filename = window-onerror9.html
window.event.lineno = 32
window.event.colno = 22