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Chrome OS is introducing a separate settings UI to control sync for
OS types like printers, OS settings, and Wi-Fi config. See
go/split-settings-sync for links to PRD and mocks.

* Add UserSelectableOsType for OS data types
* Add types for both OS preferences and printers. This CL does both
  because EnumSet requires at least 2 enum values and I want to add
  UserSelectableOsTypeSet for consistency with the browser enum set.
* Change SyncUserSettings to support the new types, as well as a
  "sync all OS types" control.
* Add new prefs for sync OS preferences and sync all OS types. There
  was an existing pref for sync printers that wasn't used -- now it
  is used.

Not everything is wrapped in #if defined(OS_CHROMEOS) because the
convention in the sync code seems to be that all ModelType values
and pref names are compiled on all platforms. However, all references
to UserSelectableOsType are ifdef'd to make it clear that "OS type"
is Chrome OS only.

Bug: 1013466
Test: added to components_unittests and sync_integration_tests
Change-Id: I1cd6f7e663c20dfb0f36c55d204b92ea19a73852
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1895755
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712216}