Perfetto traces converted with traceconv can't be imported on Firefox Profiler

STR:

1. Take an android perfetto trace. For example, open https://ui.perfetto.dev/ and click on "Open Android example", and save it on your machine (using "Download" from the menu).

2. Use [traceconv](https://perfetto.dev/docs/quickstart/traceconv) to convert the profile into a json profile:
```
curl -LO https://get.perfetto.dev/traceconv
chmod +x traceconv
./traceconv json [input proto file] [output file]
```
3. Open [Firefox Profiler](https://profiler.firefox.com/) and import the json file

The json I used can be downloaded [here](https://drive.google.com/file/d/1Etc774jnectdK6aLkrbyVtx46gMm5jpM/view?usp=sharing).

## Expected result

The profile is imported.

## Actual result

The profile fails to be imported, with the following message:

```
Error: No threads were captured in this profile, there is nothing to display.
```

(copied from bug-1968544)

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-911)