InsertNodeBeforeCommand shouldn't use confusingAndOftenMisusedAttached()

This code wants inActiveDocument().

Review URL: https://codereview.chromium.org/50563005

git-svn-id: svn://svn.chromium.org/blink/trunk@160867 bbb929c8-8fbe-4397-9dbb-9b2b20218538