Fix bug in deletion of indexed properties

Fix bug in deletion of indexed properties

The delete operator always return true in case of indexed property. It
should return false if an indexed property can't be deleted (eg.
DontDelete attribute is set or a string object is the holder).

Contributed by Peter Varga <pvarga@inf.u-szeged.hu>

BUG=none
TEST=mjsunit/delete-non-configurable

Review URL: https://codereview.chromium.org/11094021
Patch from Peter Varga <pvarga@inf.u-szeged.hu>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12736 ce2b1a6d-e550-0410-aec6-3dcde31c8c00