Convert DOMWindow to ScriptWrappable and add virtuals for DOM accessors.

This patch is split off the in-progress patch to add DOMWindow and
RemoteDOMWindow to support out-of-process iframes.

BUG=425623

Review URL: https://codereview.chromium.org/681233002

git-svn-id: svn://svn.chromium.org/blink/trunk@184538 bbb929c8-8fbe-4397-9dbb-9b2b20218538