Remove lots of stuff from NodeRenderingContext

Lots of simplification can be done to NodeRenderingContext. For example
createTextRenderer will never return null so we don't need to check for that.
We can also combine all the constructors. The only case that used the RenderStyle
version that didn't compute the parent was in Text::updateTextRenderer and this
optimization can't be buying us much since we call into NodeRenderingTraversal::parent
all over the place so saving the one call here won't matter.

Review URL: https://chromiumcodereview.appspot.com/20017008

git-svn-id: svn://svn.chromium.org/blink/trunk@154813 bbb929c8-8fbe-4397-9dbb-9b2b20218538