Move variations prefs into the variations component

These prefs are used by variations code that is targeted for componentization.

BUG=382865
TBR=thakis

Review URL: https://codereview.chromium.org/1265423003

Cr-Commit-Position: refs/heads/master@{#343661}
void VariationsSeedStore::RegisterPrefs(PrefRegistrySimple* registry) {
  registry->RegisterStringPref(prefs::kVariationsCompressedSeed, std::string());
  registry->RegisterStringPref(prefs::kVariationsSeed, std::string());
  registry->RegisterInt64Pref(prefs::kVariationsSeedDate,
                              base::Time().ToInternalValue());
  registry->RegisterStringPref(prefs::kVariationsSeedSignature, std::string());
  registry->RegisterStringPref(prefs::kVariationsCountry, std::string());
}

void VariationsSeedStore::RegisterPrefs(PrefRegistrySimple* registry) {
  registry->RegisterStringPref(prefs::kVariationsCompressedSeed, std::string());
  registry->RegisterStringPref(prefs::kVariationsSeed, std::string());
  registry->RegisterInt64Pref(prefs::kVariationsSeedDate,
                              base::Time().ToInternalValue());
  registry->RegisterStringPref(prefs::kVariationsSeedSignature, std::string());
  registry->RegisterStringPref(prefs::kVariationsCountry, std::string());
}
