Simplify DOMWindowProperty and its unregistration.

As LocalDOMWindow controls the registration lifetime of its
DOMWindowProperties, there's no need to offer a separate unregistration
mechanism. Remove it, simplify notification code + retire now unused
LocalDOMWindow back reference on DOMWindowProperty.

R=haraken,ager
BUG=

Review URL: https://codereview.chromium.org/558213002

git-svn-id: svn://svn.chromium.org/blink/trunk@181758 bbb929c8-8fbe-4397-9dbb-9b2b20218538