Consider computing traced timing for diffed sample profiles

PR #2558 adds the ability to compute traced timing for sample data. However, the case to do this with diff profiles is a bit more complicated, as we would need to compute the traced timing for POSITIVE samples, and then again, for NEGATIVE samples, then sum them together. At the implementation time, it wasn't worth it to do it. This issue is for tracking if people actually want it.

See the function `computeTracedTiming`

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-278)

Start by reviewing PR #2558 and the computeTracedTiming function, then inspect how positive and negative samples are represented in diff profiles. Done means computing traced timing separately for POSITIVE and NEGATIVE samples and summing the results, subject to confirming that this behavior is wanted.

Written by the indexing model from the issue text.