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