Only zapped used handles if a handle scope fits entirely within one handle block

This should avoid zapping the same unused handles over and over again
when using many small nested handle scopes.

BUG=none
R=mstarzinger@chromium.org, vegorov@chromium.org

Review URL: https://codereview.chromium.org/24359003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16987 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
make v8::Locker not use Isolate::GetCurrent()

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/25002004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16988 ce2b1a6d-e550-0410-aec6-3dcde31c8c00