Reland "Enable Object.observe by default"

Original Issue: https://codereview.chromium.org/183683022/

TBR=rossberg
BUG=v8:2409
LOG=Y

Review URL: https://codereview.chromium.org/189513010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19736 ce2b1a6d-e550-0410-aec6-3dcde31c8c00