Don't leak contexts in Object.observe

The Object.observe API may construct internal structures as a result of API calls. These structures can persist as long as an object that was once observed persists. This patch ensures that these structures are created in the correct context so as to avoid leaking contexts

R=verwaest@chromium.org, dcarney
BUG=

Review URL: https://codereview.chromium.org/263833007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21126 ce2b1a6d-e550-0410-aec6-3dcde31c8c00