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We rely on sync to provide the user's OS settings when setting
up a new Chromebook. However, new devices are not guaranteed
to be running the latest version of Chrome before the user's
first login (it's a poor experience to have to wait to download
a large OS update before being allowed to sign in).

As part of go/split-settings-sync we are migrating OS prefs to
new sync ModelTypes OS_PREFERENCES and OS_PRIORITY_PREFERENCES.
We need to ensure that old clients still get synced updates for
these preferences.

When split sync is disabled, register OS pref names with the
browser pref ModelTypes. This makes sync work just like it did
before the split, and allows OS prefs to always be registered
as SYNCABLE_OS_PREF, without conditionals on the flag.

When split sync is enabled, register OS pref names with the
PrefModelAssociators for both the new ModelType and the old
ModelType. When a pref is changed locally, this sends updates
to the sync server under both ModelTypes.

However, when a sync change is received from the server we
only apply the update if the change is for the new ModelType.
We made a product decision not to sync updates from old clients
to new clients. This helps avoid ping-pong updates between old
and new clients.

Bug: 1013466
Test: added to components_unittests
Test: manually verified that new client with flag enabled
  sends updates to old client (M78), but old client changes
  are ignored on new client.
  updates
Test: manually verified that client with flag disabled
  syncs updates in both directions with old client (M78).

Change-Id: I04c494938d3526c57b8eb2f9c689acb0d4b0a3ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865662
Commit-Queue: James Cook <jamescook@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708442}