The stack sampling can be abstracted to only use a reference to a `ProfileBuffer`, and the existing `locked_profiler_get_backtrace` can provide its stack-based `ProfileBuffer` that points at a heap-based `ProfileChunkedBuffer` (the one that will be stored in the returned `ProfilerBacktrace`).
And we can now add a public `profiler_capture_backtrace` that only takes a reference to a `ProfileChunkedBuffer`, and fills it with a backtrace.
This will be used by the new marker API, to optionally capture a backtrace in stack-based buffers at the user's request.
Differential Revision: https://phabricator.services.mozilla.com/D86514
2ae86bbfac