ASSERTs in recalcStyle should use parentOrShadowHostNode()

The ASSERTs in recalcStyle and recalcOwnStyle should being using
parentOrShadowHostNode() instead of null checking parentNode(). This makes
them check parent ShadowRoots and makes it clear you always have a parent
when going through this code.

Review URL: https://codereview.chromium.org/61333007

git-svn-id: svn://svn.chromium.org/blink/trunk@161751 bbb929c8-8fbe-4397-9dbb-9b2b20218538