Oilpan: move DOMWindow object to the oilpan heap.

Turn the global Window into a garbage collected object.

R=haraken@chromium.org,ager@chromium.org,wibling@chromium.org,zerny@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/211373002

git-svn-id: svn://svn.chromium.org/blink/trunk@170065 bbb929c8-8fbe-4397-9dbb-9b2b20218538