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Creates a field trial to control the SplitSettingsSync feature. The
trial is client controlled because SplitSettingsSync controls the
out-of-box experience (OOBE) sync consent dialog, which shows up
before a variations seed is available.

The trial group chosen on first run is persisted to local state prefs
and reused on subsequent runs. This keeps the in-session sync settings
UI stable between runs. Local state prefs can be reset via powerwash,
which will result in re-randomization, but this also sends the user
through the first-run flow again and they will see the appropriate
consent flow.

Bug: 1090989
Test: Went through OOBE, verified trial group is random then persists

Change-Id: I241407a47179ddeed1da06f2aa1189fac936c70f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2230771
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775709}