Revert "Reland "Enable Object.observe by default""

This reverts commit r19736 for breaking browser_tests on ChromiumOS.

TBR=rossberg@chromium.org,rafaelw@chromium.org

Review URL: https://codereview.chromium.org/195603002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19819 ce2b1a6d-e550-0410-aec6-3dcde31c8c00