Remove PROHIBITS_OVERWRITING as it is subsumed by non-configurable properties.

v8::DontDelete is set for Unforgeable properties, so just not setting PROHIBITS_OVERWRITING should be enough.

The secondary "feature" of not allowing accessors to be installed in extending objects is incorrect and confusing, given that it only applies to accessors but not to regular properties:
Object.defineProperty({__proto__:window}, "location", { value: 10 })
works where
Object.defineProperty({__proto__:window}, "location", { get: function() {} })
doesn't work.

LOG=y
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/306203002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21596 ce2b1a6d-e550-0410-aec6-3dcde31c8c00