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