Perform context disposal garbage collections only if contexts disposals happen at a high rate.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/710603003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25208 ce2b1a6d-e550-0410-aec6-3dcde31c8c00