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Fix a bug where an existing user could auto-update to M85, get
randomized into the SplitSettingsSync field trial, but end up
with OS sync disabled. This results in apps, OS preferences,
Wi-Fi configs and wallpaper not syncing.

The root cause is that when we migrate their per-data-type sync
prefs (e.g. apps -> OS apps, preferences -> OS preferences) we
were not enabling the global OS sync feature pref. That pref
has a "safe" default value of false, so OS sync was never enabled.

This does not affect new users, because they see the OOBE sync
consent dialog, which enables OS sync if the user opts in.

This CL also fixes migration of the OS wallpaper sync pref, which
should be set if the user was previously syncing both apps and
themes.

Some of the regression came from this CL, though migration wasn't
quite right before either:
https://chromium-review.googlesource.com/c/chromium/src/+/2197489

We noticed this in M85 because that's the first milestone that
contains the SplitSettingsSync trial.

Bug: 1116046
Test: added to unit_tests

Change-Id: Ie40fc2b4bdc194a16b7b756c82f1e2bc258c60ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354724
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798307}