chromeos: Enable feature SplitSettingsSync by default

This is the "V1" feature. It:
* Creates a new "Sync your settings" section in Chrome OS settings
* Provides a separate toggle for OS preferences, distinct from browser
  preferences
* Moves "Apps" and "Wallpapers" to OS settings
* Makes the OS ModelTypes run in sync transport mode, controlled by a
  master pref for the OS sync feature

This also unblocks the launch of Wi-Fi configuration sync.

Bug: 1013466
Change-Id: Ie203fc96a1cf6a812d4c2b1a7c994d2978964b56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2137999Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757043}

chromeos/constants/chromeos_features.cc

@@ -330,7 +330,7 @@ const base::Feature kSmartDimModelV3{"SmartDimModelV3",
 // For example, the user could choose to sync OS settings but not browser
 // settings.
 const base::Feature kSplitSettingsSync{"SplitSettingsSync",
-                                       base::FEATURE_DISABLED_BY_DEFAULT};
+                                       base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Introduces a new OOBE dialog for the OS sync feature. Uses the same browser
 // sync consent dialog as Windows/Mac/Linux. Allows the user to fully opt-out of