Fix handling of foreign callbacks in DefineOwnProperty.

Fix handling of foreign callbacks in DefineOwnProperty.

We use foreign callbacks to make some properties shadow internal values
but still behave as data properties from within JavaScript. This means
when a value is passed to Object.defineProperty() on such a property,
it should update the internal value instead of redefinind the property
and destroying the shadowing.

R=rossberg@chromium.org
BUG=v8:1530
TEST=mjsunit/regress/regress-1530,test262/S15.3.3.1_A4

Review URL: http://codereview.chromium.org/8996008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10279 ce2b1a6d-e550-0410-aec6-3dcde31c8c00