CPU data isn't used on merged threads

Example profile: https://share.firefox.dev/3ov4kcQ

Notice how the activity graph seems capped at 50%, because the "other" thread used in the merged thread is mostly empty.

Edit: This issue is updated to reflect other merged treads and not active tab view.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-515)

Start with the linked example profile and the activity graph behavior described in the issue. Trace how merged threads contribute CPU data, including the mostly empty “other” thread, and compare the graph with the updated scope for merged threads rather than active-tab view. Done means CPU usage from merged threads is represented instead of leaving the graph capped at 50%.

Written by the indexing model from the issue text.