Add d8 functionality for switching between realms (a.k.a. contexts), 2nd try

The only difference to the previous implementation (https://codereview.chromium.org/14295011/) is in patch set 2.

R=yangguo@chromium.org
BUG=

Review URL: https://codereview.chromium.org/14404005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14373 ce2b1a6d-e550-0410-aec6-3dcde31c8c00