Move CopyReplaceDescriptors to private

BUG=
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/239203002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20793 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
LazyInstance should be thread safe by default.

The only ways we use this class require it, as multiple isolates can race
to initialize data structures. This showed up as intermittant failures on
nosnap build bots.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/238973004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20794 ce2b1a6d-e550-0410-aec6-3dcde31c8c00