Profiling sources of JS garbage no longer works?

Hi,

I am trying to use Firefox Profiler to benchmark code lines that allocate JS garbage. To do that, I enable the option `"JS Allocations"` in the profiler and do a run of the page that does allocations.

However, when I view the profile, there are no allocations recorded: https://share.firefox.dev/3oYdAto

I wonder if the feature might have regressed?

In the past I recall the capture sampling was probabilistic, and defaulted to capturing 5% of all JS allocations. There was a pref I would set to up the rate to capturing all 100% allocations. Is that probabilistic capturing still a thing? I am benchmarking a WebAssembly page, and it might be possible that there aren't that many JS allocations overall (and the intent is that since it is wasm, there shouldn't practically be almost any, hence I am profiling to see JS call sites where there would be allocations)

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-647)