message: Use %zu format specifier for size_t printf argument.
  This fixes -Wformat error in 32-bit Android build.
diff: @@ -200,7 +200,7 @@ Status StatusGroup::as_status() const {
  const std::pair<error::Code, int>& b) { return a.second < b.second; });
  fmt.push_back(
  - strings::Printf("Combined status information from %lu operations:\n",
  + strings::Printf("Combined status information from %zu operations:\n",
  num_ok_ + children_.size()));
  for (const auto& p : count_vec) {
  