Get rid of the ZoneScopeMode.

No one is using the DONT_DELETE_ON_EXIT mode for ZoneScopes anymore, so
we can safely assume that all ZoneScopes are DELETE_ON_EXIT now.

R=danno@chromium.org
BUG=

Review URL: https://codereview.chromium.org/17826004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15336 ce2b1a6d-e550-0410-aec6-3dcde31c8c00