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The low entropy source is reported as a variation ID offset from
3320978. For example, a low entropy source value of 10 is reported
as variation ID 3320988.

The low entropy source is a locally-generated value in the range of
0-7999 that's used by Chrome to derive the variation ids included
in the X-Client-Data header. Since other variation ids have been
selected based on this value, this does not expose any additional
entropy in the header given enough variation ids.

Note: 8000 possible values is under 2^13, therefore having 13+ A/B
variations would carry the same amount of entropy. A typical
Chrome client has more than 13 variation ids reported.

Adding this value allows for retrospective A/A tests for
validating that there's no existing bias between two randomized
groups of clients for a later A/B study.

Bug: 1134444
Change-Id: Ia1af76f469cb2e3e25b03884abc4974cec237977
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461906
Commit-Queue: Karthika Pai <karthikapai@google.com>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825962}