Repeated invocations of processThreadCPUDelta during symbolication take up more time than necessary

Profile: https://share.firefox.dev/2X0vayw

Every time symbolication updates the thread, `processThreadCPUDelta` is called again for the modified profile.
It seems like this is something that could be done only once. We could bake the guarantees that `processThreadCPUDelta` makes into the processed profile format. That means, instead of calling it from a selector, we would call it in these two places:

* During profile processing, and
* In an upgrader for the processed profile format.

It's not taking a ton of time, and symbolication is slow already, but it's one of many things that adds up to the slowness and it might be easy to fix.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-426)