Oilpan: Remove RefCounted from DOMWindow in the Oilpan build.

Noticed in passing that DOMWindow was still RefCountedGarbageCollected.
It doesn't have to be.

R=haraken@chromium.org, oilpan-reviews@chromium.org

Review URL: https://codereview.chromium.org/335183003

git-svn-id: svn://svn.chromium.org/blink/trunk@176594 bbb929c8-8fbe-4397-9dbb-9b2b20218538