Add a use counter API

This lets embedders track certain features of v8 and the number of times
they are used

BUG=none
R=svenpanne@chromium.org, marja@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/346233002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21925 ce2b1a6d-e550-0410-aec6-3dcde31c8c00