Reland r173864 "Remove obsolete didClearWindowObject API and bogus FIXME"

> The FIXME was wondering about why the inspector didn't take a certain
> codepath - the reason is that the inspector code only cares about the
> document property being reset, not about the v8 global object being
> created.
>
> BUG=none
> R=dcarney@chromium.org
>
> Review URL: https://codereview.chromium.org/279153002

BUG=none
TBR=dcarney@chromium.org

Review URL: https://codereview.chromium.org/290133002

git-svn-id: svn://svn.chromium.org/blink/trunk@174160 bbb929c8-8fbe-4397-9dbb-9b2b20218538