Test that window.onerror is called on window object when there is a syntax error in attribute handler. Bug 70991.

Main frame window.onerror: Uncaught SyntaxError: Unexpected token % at window-onerror-syntax-error-in-attr.html:10
Main frame window.onerror: Uncaught SyntaxError: Unexpected token % at window-onerror-syntax-error-in-attr.html:36
Main frame window.onerror: Uncaught SyntaxError: Unexpected token % at window-onerror-syntax-error-in-attr.html:36
Button 1 Button 2 Button 3