fast/forms/autofocus-in-sandbox-with-allow-scripts.html should wait for onload

autofocus doesn't happen until onload once the parser lazy attaches, so
alert(document.activeElement) in the iframe in this test doesn't work. Instead
do the alert inside onload.

It turns out making the parser lazy attach makes our autofocus behavior match
other browsers as well (moving it to onload).

BUG=276687
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/23332014

git-svn-id: svn://svn.chromium.org/blink/trunk@156525 bbb929c8-8fbe-4397-9dbb-9b2b20218538