Original issue: https://codereview.chromium.org/15331002/

Broke mozilla/js1_5/Array/regress-451483.js, which ends up attempting to call %IsObserved() on a non-object.

IsObserved now checks for JSReceiver and returns false rather than crashing.

R=adamk@chromium.org, rossberg@chromium.org

Review URL: https://codereview.chromium.org/15777007

Patch from Rafael Weinstein <rafaelw@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14888 ce2b1a6d-e550-0410-aec6-3dcde31c8c00