Add a device policy to DeviceSettingsProvider.

This was missed in a previous CL crrev.com/c/1200524.

BUG=chromium:879737
TEST=Browsertests passed with code that uses this setting.

Change-Id: I6ca354c0a781a16d17fb2be5c12203209992e25b
Reviewed-on: https://chromium-review.googlesource.com/1211731Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Tim Zheng <timzheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589588}

chrome/browser/chromeos/settings/device_settings_provider.cc

@@ -84,6 +84,7 @@ const char* const kKnownSettings[] = {
     kDevicePrintersConfigurations,
     kDevicePrintersWhitelist,
     kDeviceQuirksDownloadEnabled,
+    kDeviceUnaffiliatedCrostiniAllowed,
     kDeviceWallpaperImage,
     kDisplayRotationDefault,
     kExtensionCacheSize,