Also, add user time into heap sample begin events to make '--log-gc' flag alone sufficient for producing heap logs (previously, samples times were extracted from scavenge events which are only logged with '--log' flag).

Review URL: http://codereview.chromium.org/149611

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2461 ce2b1a6d-e550-0410-aec6-3dcde31c8c00