Make it a part of V8 API GetStackSample function.
Also expose external_callback_entry in SampleInfo to break dependency
of clients on internal V8 structures.

BUG=v8:4789

Committed: https://crrev.com/70acfe39c07322144f5fe9b40bb584a8b1099ffd
Review-Url: https://codereview.chromium.org/2007343003
Cr-Original-Commit-Position: refs/heads/master@{#36831}
Cr-Commit-Position: refs/heads/master@{#36836}