Move assertions into notifyNodeInsertedInternal

Put the assertions inside the mehtod with the loop so it's clear that
this method is protected.

Review URL: https://codereview.chromium.org/304853002

git-svn-id: svn://svn.chromium.org/blink/trunk@175025 bbb929c8-8fbe-4397-9dbb-9b2b20218538