Stop passing dummy ParentDetails to NodeRenderingTraversal::parent()

Stop passing dummy ParentDetails to NodeRenderingTraversal::parent(). It
handles a null pointer just fine so just make the argument optional and
default it to 0.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/410733004

git-svn-id: svn://svn.chromium.org/blink/trunk@178712 bbb929c8-8fbe-4397-9dbb-9b2b20218538