Thread merging doesn't update string indexes in marker fields

[mstange]

Steps to reproduce:

1. Load https://share.firefox.dev/3T3UduQ
2. Move your mouse over the markers and see that the tooltip displays normal strings.
3. Shift-click the TaskController thread, so that you have a multi-thread selection.
4. Move your mouse over the markers again.

Expected results: The same strings as before should be shown in the tooltip.

Actual results: The tooltip displays nonsense strings with hex addresses.

┆Issue is synchronized with this Jira Task

[julienw]

Ah interesting, this should happen somewhere in the function https://github.com/firefox-devtools/profiler/blob/6a8c668e51c24f697bd21a94f0b2e02767019e02/src/profile-logic/merge-compare.js#L1182. We're already doing something similar hardcoded for CompositorScreenshot markers, but here we need to look at the original markerSchema. The general solution should also handle merging the markerSchemas, but we can start with the easy case of merging 2 threads of the same profile (so with the same markerSchema for both).