As several pages can run in a single V8 instance, it is possible to
have functions from different security contexts intermixed in a single
CPU profile.  To avoid exposing function names from one page to
another, filtering is introduced.

The basic idea is that instead of capturing return addresses from
stack, we're now capturing JSFunction addresses (as we anyway work
only with JS stack frames.)  Each JSFunction can reach out for
context's security token. When providing a profile to a page, the
profile is filtered using the security token of caller page. Any
functions with different security tokens are filtered out (yes, we
only do fast path check for now) and their ticks are attributed to
their parents.

Review URL: http://codereview.chromium.org/2083005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4673 ce2b1a6d-e550-0410-aec6-3dcde31c8c00