[starlark] Expose CaptureStacktrace as public Go function.
[starlark] Expose CaptureStacktrace as public Go function.

This would allow to capture stack traces in Go implementation of various builtin
methods.

R=ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org
BUG=833946

Change-Id: I7ccc5d6604697d046696d6cc7d464f9fccb9edf4
Reviewed-on: https://chromium-review.googlesource.com/c/1357256
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go