The functions of strong classes are born non extensible. But, when
the class is created we need to add an own private symbol representing
the [[HomeObject]] slot in the spec.
Like for the hidden_string property, we allow adding private own
symbols to non extensible objects.
BUG=v8:4077
LOG=N
R=rossberg@chromium.org
Review URL: https://codereview.chromium.org/1138603003
Cr-Commit-Position: refs/heads/master@{#28447}
