By passing around a Zone object explicitly we no longer need to do a
TLS access at the sites that allocate memory from the current Zone.

BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/10534006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11761 ce2b1a6d-e550-0410-aec6-3dcde31c8c00