RenderNamedFlowThread should not create NodeRenderingContext objects

RenderNamedFlowThread is using NodeRenderingContext to get the original
parent renderer, but should just be using NodeRenderingTraversal::parent
instead (which is what NodeRenderingContext does internally anyway).

Review URL: https://chromiumcodereview.appspot.com/22623003

git-svn-id: svn://svn.chromium.org/blink/trunk@155722 bbb929c8-8fbe-4397-9dbb-9b2b20218538