If CPU deltas are given as times (as opposed to cycle counts), 100% CPU should be based on cpuDelta == timeDelta, independently of the largest observed delta

If CPU deltas are given in microseconds, but all samples have cpuDelta < sampleTimeDelta, then we display incorrect CPU percentages: We make it look like the CPU usage was higher than it actually was, by making the "largest observed CPU usage" 100%, even if it wasn't 100%.

Profile with 50% CPU usage throughout the entire profile: https://share.firefox.dev/3IjJO76

Expected results: We should display 50% CPU.
Actual results: We display 100% CPU.

Profile with 50% CPU usage through most of the profile, and a 100% sample at the end: https://share.firefox.dev/3Gee8OG
This one actually displays 50%.

I generated these profiles with [this Rust program](https://gist.github.com/mstange/c1c2b61442b165f28fa0384e9eab820b).

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-476)