Simplify NodeRenderingTraversal::parentElement() a bit

Simplify NodeRenderingTraversal::parentElement() a bit by not creating a dummy
ParentDetails object. NodeRenderingTraversal::parent() does not require passing
a ParentDetails object in parameter.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/482873002

git-svn-id: svn://svn.chromium.org/blink/trunk@180493 bbb929c8-8fbe-4397-9dbb-9b2b20218538