Verifies that mouse events are not fired on deleted nodes (crbug.com/515921)

On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".

outer received mouseover
outer received mouseenter
outer received mouseout
inner received mouseover
inner received mouseenter
removed inner
TODO(crbug.com/276329): We are missing a mouseout here!
TODO(crbug.com/276329): We are missing a mouseleave here!
outer received mouseover
outer received mouseenter
PASS successfullyParsed is true

TEST COMPLETE