Revisit how we select the thread at first load

[julienw]

Currently we try to select the first non-hidden GeckoMain thread for a content process. Historically this was because we were targeting more web developers, and this was hitting more interesting processes for this use case. But now the preset "web developer" is more suited for this use case, and we should probably select the first visible track, as this is less surprising.

Here is an example: https://profiler.firefox.com/public/dd9tc1rk38edcdt77xjvca7dxq0jdtz7gpmp6fr/calltree/

This happens in src/profile-logic/tracks.js (look for initializeSelectedThreadIndex).

┆Issue is synchronized with this Jira Task

[trizist]

I need help

[canova]

We have made significant improvements to this since this issue. Namely:

• We changed the order of tracks to use the activity score.
• Changed the default visible threads to use the activity score.
• Select the default thread by picking the most active non-parent process track.

Considering all these, I think this issue is outdated and we can close it.