[User Education] Make command line switch public

It's been requested to have a public declaration for the "disable rate
limiting" command line switch, so this moves it to the .h file.

Change-Id: I95ae5af89724e7bda9f10fc999157183dd095b50
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5675852
Auto-Submit: Dana Fried <dfried@chromium.org>
Reviewed-by: Dmitry Vykochko <vykochko@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Dmitry Vykochko <vykochko@google.com>
Cr-Commit-Position: refs/heads/main@{#1323215}