Track up to 5 stable maps as field type.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/240813002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20831 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Some MaybeHandles related cleanups.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/240443005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20832 ce2b1a6d-e550-0410-aec6-3dcde31c8c00