CcTest::isolate_used_ is used from multiple threads, make it atomic

BUG=none
R=svenpanne@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/637263003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24460 ce2b1a6d-e550-0410-aec6-3dcde31c8c00