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

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

Committed: https://code.google.com/p/v8/source/detail?r=21596

[Toon Verwaest, 2014-05-30 18:57:31]

PTAL. This will require a tiny change on the blink side, which can land before
this CL lands.

[dcarney, 2014-06-02 07:00:06]

lgtm

[Toon Verwaest, 2014-06-02 11:02:17]

Committed patchset #2 manually as r21596 (presubmit successful).