Prevent changes to hidden properties from being observable via Object.observe

This addresses the leak that mstarzinger points out (https://codereview.chromium.org/26390003/) and includes the test.

Note that this adds a test that observing changes to the empty-string property remains possible.

BUG=
R=mstarzinger@chromium.org, rossberg@chromium.org

Review URL: https://codereview.chromium.org/26592012

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17257 ce2b1a6d-e550-0410-aec6-3dcde31c8c00