Added histogram to track distribution of requested idle notifications.

BUG=397026
LOG=n
R=rmcilroy@chromium.org

Review URL: https://codereview.chromium.org/420753002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22632 ce2b1a6d-e550-0410-aec6-3dcde31c8c00