Get rid of ZoneScope completely.

There's no need to differentiate between an actual Zone and its
scope. Instead we bind the lifetime of the Zone memory to the
lifetime of the Zone itself, which is way easier to understand
than having to dig through the code looking for zone scopes.

Depends on https://codereview.chromium.org/17826004/

R=danno@chromium.org
BUG=

Review URL: https://codereview.chromium.org/17827005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15337 ce2b1a6d-e550-0410-aec6-3dcde31c8c00