The content process of a page load may be hidden if the page uses very little JavaScript

I saw a case yesterday of trying to profile a page load, and when the profile was captured, only the parent process was visible by default.

When showing all tracks, the content process of the page load was actually present in the profile, but it got hidden by default. I think the reason is that the page used almost no JavaScript and loaded very quickly, so it had very few active samples (Julien looked up what we do in the code, and concluded that we hide the track if less than 5% of the samples are non-Idle, which might have been the case in this profile).
Visually, I could identify the relevant content process because it had several network requests, and page load markers in the network track. I think we should use this information to decide to not hide a process.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-436)