Merged r14902 into trunk branch.

Fix non-idempotent modification in JSObject::AddFastProperty.

R=rossberg@chromium.org
TEST=mozilla/ecma/Array/15.4.5.2-2

Review URL: https://codereview.chromium.org/16256002

git-svn-id: https://v8.googlecode.com/svn/trunk@14904 ce2b1a6d-e550-0410-aec6-3dcde31c8c00