Support idle time in CPU profiler

This change provides an API for the embedder to tell CPU profiler if it is idle or busy with some task. This way we can discriminate between idle time and some native code execution.

BUG=268947
R=alph@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/22412003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16109 ce2b1a6d-e550-0410-aec6-3dcde31c8c00