Allow recording individual samples in addition to the aggregated CPU profiles

CPU profiler API is extended with methods that allow to retrieve individual samples from profile. Each sample is presented as a pointer to a node in the top-down profile tree. The samples will let us tie JS performance to time.

BUG=None

Review URL: https://codereview.chromium.org/12919002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13980 ce2b1a6d-e550-0410-aec6-3dcde31c8c00