Fix data races and leaks related to v8::Lockers

BUG=v8:3618
R=ishell@chromium.org, svenpanne@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/637263002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24453 ce2b1a6d-e550-0410-aec6-3dcde31c8c00