Inside Document use renderView() instead of renderer()

renderView() doesn't contain a branch like renderer() does and also
returns a more specific type, we should use it whenever possible.

Review URL: https://codereview.chromium.org/53843002

git-svn-id: svn://svn.chromium.org/blink/trunk@161020 bbb929c8-8fbe-4397-9dbb-9b2b20218538