DOMWindow::clearDocument should not use confusingAndOftenMisusedAttached()

This caller wants Document::isActive() instead.

Review URL: https://codereview.chromium.org/53093006

git-svn-id: svn://svn.chromium.org/blink/trunk@161002 bbb929c8-8fbe-4397-9dbb-9b2b20218538