Use [ImplementedAs=defaultStatus] for Window.defaultstatus.

This CL use [ImplementedAs=defaultStatus] for Window.defaultstatus
and remove unnecessary code from DOMWindow.
Because 'defaultstatus' is an alias of defaultStatus.

Review URL: https://codereview.chromium.org/69953018

git-svn-id: svn://svn.chromium.org/blink/trunk@161994 bbb929c8-8fbe-4397-9dbb-9b2b20218538