Improve comment in perf_test.h.
Improve comment in perf_test.h.

I think these functions are so hard to understand, so I tried to
make an as grounded example as possible.

Bug: chromium:1029452
Change-Id: I5d4284bc15b39cb94ba42d2c483a619ecf42fb91
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168945
Reviewed-by: Artem Titov <1e27363ccc366a7ff0fdde48204d68d5ebfeccc5@webrtc.org>
Commit-Queue: Patrik Höglund <b486136ee68458b09c92c86c39f451c87d6ef6a1@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#30590}

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