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This keeps them out of the public pref_names.h header and makes it
more obvious that they are obsolete.

This reveals that the Android ChromeBackupAgent has been backing up
empty prefs since 1/2019. Stop backing up those prefs. Based on my
reading of the backup and restore code this should work OK.
The restore code already checks to make sure all the prefs being
restored are still valid names.

Bug: 906611

Change-Id: I962ddc0f52f06f38247647bffeabba85a0721ef2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1929974
Commit-Queue: Marc Treib <treib@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718642}