fix($rootScope): workaround for Chrome's memleak

Under certain circumstances chrome fails to GC scopes
because of buggy optimizations and caching. Nulling out
references to (not from!) other scopes helps Chrome to
realize that this object should be GC-ed.

This is really just a workaround as the real problem needs
to be fixed in Chrome.

See discusstion at:
https://github.com/angular/angular.js/issues/1313#issuecomment-10378451

And chrome bug at:
https://code.google.com/p/v8/issues/detail?id=2073

Closes #1313