More simplifcation of NodeRenderingContext

NodeRenderingContext has too many responsibilities and has become a dumping ground
for utility methods. We can replace callers of several of the methods with
NodeRenderingTraversal and can also eliminate the m_flowThread member which was
never read.

I also cleaned up callers of NodeRenderingTraversal::parent that were passing details when
it wasn't needed.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/17147002

git-svn-id: svn://svn.chromium.org/blink/trunk@154819 bbb929c8-8fbe-4397-9dbb-9b2b20218538