Simplify RenderObject::containerForRepaint Regions code.

-We no longer support seamless iframes, so remove the code that
deals with the flow thread being in a separate document.
-Simplify the remaining logic a bit. No need for the ternary.

Review URL: https://codereview.chromium.org/237843002

git-svn-id: svn://svn.chromium.org/blink/trunk@171583 bbb929c8-8fbe-4397-9dbb-9b2b20218538