DOMWindow::installNewDocument should not check confusingAndOftenMisusedAttached()

It doesn't make sense to check the document for being attached here, it
was just created above, there's no way for it to have been attached already.

Document::attach contains an ASSERT that will ensure that this is the case
already.

Review URL: https://codereview.chromium.org/48293005

git-svn-id: svn://svn.chromium.org/blink/trunk@161013 bbb929c8-8fbe-4397-9dbb-9b2b20218538