Convert function.caller to API-style accessor.

BUG=
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/256803010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21021 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Tighten OpenHandle's extra checks.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/240113010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21022 ce2b1a6d-e550-0410-aec6-3dcde31c8c00