Update getparts benchmark to enable per-part measurement

This makes the getparts benchmark able to measure the call overhead of
getParts() as a function of the number of parts.

No functional change to code.

See https://docs.google.com/document/d/1wSbBV6tS-KLlHWxaK_FmEJFOwkY-pb8_YDoFJgDrbMg/edit#heading=h.suz4c63mrpt9

Bug: 1453291
Change-Id: I3078ecb3b923da0fefe113b4d9990f72b3a61f83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4926790
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1207858}