Oilpan: turn DOMWindow into a heap supplementable.

With DOMWindow now a garbage collected object, follow up and also turn
it into a heap-residing Supplementable, along with its Supplements.

R=haraken@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/212933005

git-svn-id: svn://svn.chromium.org/blink/trunk@170146 bbb929c8-8fbe-4397-9dbb-9b2b20218538