Fix non-idempotent modification in JSObject::AddFastProperty.

R=verwaest@chromium.org
BUG=mozilla/ecma/Array/15.4.5.2-2

Review URL: https://codereview.chromium.org/15955010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14902 ce2b1a6d-e550-0410-aec6-3dcde31c8c00