Harden %SetIsObserved with RUNTIME_ASSERTs

Now throws if its argument is already observed, or if the argument is
the global proxy.

BUG=371782
LOG=Y
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/274163002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21256 ce2b1a6d-e550-0410-aec6-3dcde31c8c00