Fix evaluation order problems in Object.observe hooks.

R=rossberg@chromium.org
TEST=gcmole

Review URL: https://codereview.chromium.org/14493012

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14432 ce2b1a6d-e550-0410-aec6-3dcde31c8c00