Fix assert in Runtime_SetIsObserved.

R=rafaelw@chromium.org
BUG=chromium:352664
LOG=N

Review URL: https://codereview.chromium.org/204493002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20440 ce2b1a6d-e550-0410-aec6-3dcde31c8c00