Remove existingWindowShellWorkaroundWorld

This was used to notify the inspector only of events originating from
the FrameLoader. Instead, do this explicitly

BUG=none
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/137983002

git-svn-id: svn://svn.chromium.org/blink/trunk@165056 bbb929c8-8fbe-4397-9dbb-9b2b20218538