Only notify the inspector code when the main world window object was cleared

It just bails out early anyways.

BUG=none
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/137883004

git-svn-id: svn://svn.chromium.org/blink/trunk@165863 bbb929c8-8fbe-4397-9dbb-9b2b20218538