Add map transition for observed objects

This patch enables objects to undergo a single transition when they become observed, avoiding the need to create a new map for every observed objects.

Observed objects which become unobserved does not cause another map transition and unobserved does not clear the observed bit on the map. The unobserved object.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/18221006

Patch from Rafael Weinstein <rafaelw@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15650 ce2b1a6d-e550-0410-aec6-3dcde31c8c00