Test that window.onerror is called on window object when there is an exception(excluding syntax errors) in attribute handler. Bug 70991.

Main frame window.onerror: TypeError: 'null' is not an object (evaluating 'null.m') at window-onerror-exception-in-attr.html:10
Main frame window.onerror: TypeError: 'null' is not an object (evaluating 'null.m') at window-onerror-exception-in-attr.html:36
Main frame window.onerror: TypeError: 'null' is not an object (evaluating 'null.m') at window-onerror-exception-in-attr.html:36
Button 1 Button 2 Button 3