Stop passing NodeRenderingContext except in textRendererIsNeeded

We don't need the NodeRenderingContext in childShouldCreateRenderer or rendererIsNeeded,
we only need the node or the style so pass them explicitly. This makes textRendererIsNeeded
the only place we pass the NodeRenderingContext. Future patches should be able to clean that
up allowing NodeRenderingContext to only be the creator of renderers and not a multi-purpose
bag of things.

This also changes both methods to take a Node& or a RenderStyle& instead of pointers so
users of these APIs know the values can never be null.

Review URL: https://chromiumcodereview.appspot.com/23956012

git-svn-id: svn://svn.chromium.org/blink/trunk@157424 bbb929c8-8fbe-4397-9dbb-9b2b20218538