Use install_static::GetAppGuid instead of the hardcoded string in BrandcodeConfigFetcher.

Bug: 769756
Change-Id: Ifdcb0a5145ffad1d563562e2b2ea2390ff074cdc
Reviewed-on: https://chromium-review.googlesource.com/1213178
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590275}
std::string XmlConfigParser::XMLCharToString(const xmlChar* value) {
  return std::string(reinterpret_cast<const char*>(value));
}

std::string XmlConfigParser::XMLCharToString(const xmlChar* value) {
  return std::string(reinterpret_cast<const char*>(value));
}
