Implement tagging of profiler log event blocks.

Implement tagging of profiler log event blocks.

This change allows to associate integer tags with blocks of profiler
log events, and repeat calls to 'ResumeProfiler' / 'PauseProfiler' in
order to establsh nested (not necessary properly nested) blocks. By
supporting this, we will be able to match WebInspector's CPU profiler
abilities in DevTools.

I also refactored some testing code.

Review URL: http://codereview.chromium.org/619004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3889 ce2b1a6d-e550-0410-aec6-3dcde31c8c00