[autofill] Add a RandomizedEncoder class.

This CL adds a randomized encoder class. Instances of this class will
be used to encode autofill form and field metadata using differential
privacy, specifically randomized responses.

Bug: 850606
Change-Id: If0600595dafc6d3233674b92f40e3d092b2d5381
Reviewed-on: https://chromium-review.googlesource.com/c/1265832
Commit-Queue: Roger McFarlane <rogerm@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598775}
5 files changed