cros: Whitelist delayed registration of Ash's foreign syncable prefs

Use PrefRegistrySyncable's new whitelist for delayed pref registration.
(there's a race between ash prefs connecting to chrome and sync init)
Avoids PrefServiceSyncable::AddRegisteredSyncablePreference DCHECKs.

See the bug and earlier patch sets for debug code to trigger the race:
1) Build chromeos=1 on linux desktop with this patch applied
2) Run chrome --user-data-dir=/tmp/foo --login-manager
3) Go through setup, sign in to an existing account
4) Launch chrome again, sign in quickly, wait ~10s for a DCHECK

Bug: 788918
Change-Id: Ib505194900b34154b3896373b93c951a6eeb8715
Reviewed-on: https://chromium-review.googlesource.com/1012788Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563037}

chrome/browser/chromeos/preferences.cc

@@ -109,6 +109,21 @@ void TryMigrateToResolveTimezoneByGeolocationMethod(PrefService* prefs) {
                     static_cast<int>(method));
 }
 
+// Whitelist synable preferences that may be registered after sync system init.
+void WhitelistLateRegistrationPrefsForSync(
+    user_prefs::PrefRegistrySyncable* registry) {
+  // These foreign syncable preferences are registered asynchronously by Ash,
+  // perhaps after sync system initialization. Whitelist these prefs so that any
+  // values obtained via sync before the prefs are registered will be stored.
+  const char* const kAshForeignSyncablePrefs[] = {
+      ash::prefs::kEnableAutoScreenLock,      ash::prefs::kEnableStylusTools,
+      ash::prefs::kLaunchPaletteOnEjectEvent, ash::prefs::kShelfAlignment,
+      ash::prefs::kShelfAutoHideBehavior,     ash::prefs::kTapDraggingEnabled,
+  };
+  for (const auto* pref : kAshForeignSyncablePrefs)
+    registry->WhitelistLateRegistrationPrefForSync(pref);
+}
+
 }  // namespace
 
 Preferences::Preferences()
@@ -159,6 +174,8 @@ void Preferences::RegisterPrefs(PrefRegistrySimple* registry) {
 // static
 void Preferences::RegisterProfilePrefs(
     user_prefs::PrefRegistrySyncable* registry) {
+  WhitelistLateRegistrationPrefsForSync(registry);
+
   std::string hardware_keyboard_id;
   // TODO(yusukes): Remove the runtime hack.
   if (IsRunningAsSystemCompositor()) {