Fix CurrentMapForDeprecated() to return MaybeHandle instead of a null handle.

Also fix TryMigrateInstance() to return bool instead of the parameter or
a null handle.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/251683003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20996 ce2b1a6d-e550-0410-aec6-3dcde31c8c00