Allow configuring CPU profiler sampling interval using public API

The only way to change it at the moment is using a command line flag. We are going to add a setting to Chrome DevTools which would allow chaning default interval and that requires proper v8 API.

BUG=v8:2814
R=bmeurer@chromium.org, loislo@chromium.org

Review URL: https://codereview.chromium.org/23902004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16525 ce2b1a6d-e550-0410-aec6-3dcde31c8c00