Align RenderObject's containingBlock / container

The 2 functions work similarly but they had a lot of tiny
differences for no good reason. This change makes them closer
in intended behavior and adds more code sharing.

While touching the code, I removed some very old comment that
didn't match the current code.

Review URL: https://codereview.chromium.org/64913004

git-svn-id: svn://svn.chromium.org/blink/trunk@162524 bbb929c8-8fbe-4397-9dbb-9b2b20218538