InsertionPoint::parseAttribute should not call confusingAndOftenMisusedAttached()

This check doesn't make any sense. Either we're not in the document and
setNeedsStyleRecalc() doesn't do anything, or we are in the document and
so we're attached.

Review URL: https://codereview.chromium.org/49973002

git-svn-id: svn://svn.chromium.org/blink/trunk@160775 bbb929c8-8fbe-4397-9dbb-9b2b20218538