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

git-svn-id: svn://svn.chromium.org/blink/trunk@173864 bbb929c8-8fbe-4397-9dbb-9b2b20218538