Constify the rendering/ selection code

Most of the code wasn't touching |this| or the renderers but
they were not marked as const.

This is needed for another change that I have.

Review URL: https://codereview.chromium.org/566973002

git-svn-id: svn://svn.chromium.org/blink/trunk@181929 bbb929c8-8fbe-4397-9dbb-9b2b20218538