Tighten up Object.observe code to ASSERT that it never deals with globals

After r21126, Object.observe no longer allows observing the global proxy
object. This patch replaces codepaths that used to handle that case with
asserts showing that no such observation happens.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/261773006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21151 ce2b1a6d-e550-0410-aec6-3dcde31c8c00