FRE: Improve comments in GetFRERequirement

Make GetFRERequirement easier to understand by explaining why the serial
number is checked. The new comment is based on the original comment from
https://chromium.googlesource.com/chromium/src/+blame/87b65e9b67790efd6aaa2201bbee4485f9015dca/chrome/browser/chromeos/login/enrollment/auto_enrollment_controller.cc#59

Bug: none
Change-Id: I65202c838fd11f38aa7543954f7451cbf2bfdf7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1571698
Reviewed-by: Igor <igorcov@chromium.org>
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652120}
1 file changed