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}
std::string ListValueToString(const base::ListValue& list_value) {
  std::string json;
  JSONStringValueSerializer serializer(&json);
  serializer.set_pretty_print(true);
  serializer.Serialize(list_value);
  return json;
}

std::string ListValueToString(const base::ListValue& list_value) {
  std::string json;
  JSONStringValueSerializer serializer(&json);
  serializer.set_pretty_print(true);
  serializer.Serialize(list_value);
  return json;
}
