Clear() is rather wasteful for ZoneLists, because it discards
all previously allocated memory, making it impossible to reuse
until the whole Zone is released. Better use Rewind(0) in this
case, which just resets the length to zero.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/302783007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21545 ce2b1a6d-e550-0410-aec6-3dcde31c8c00