Change __defineGetter__ and __defineSetter__ to respect non-configurable.

Change __defineGetter__ and __defineSetter__ to respect non-configurable.

This makes us compatible with firefox. Earlier on we were somehow
compatible with safari - which will allow defining a getter even when
an existing getter is present and non-configurable. We would, however,
in addition to overwriting the getter also change configurable to
true. The approach used by firefox seems much more sound, i.e., why
should it be possible to use __defineGetter__ or __defineSetter__ to
overwrite a non-configurable getter or setter respectively.

I will file a bug on the webkit bugtracker.

Review URL: http://codereview.chromium.org/6658037

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7143 ce2b1a6d-e550-0410-aec6-3dcde31c8c00