### Preflight Checklist

* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

### Issue Details

* **Electron Version:**
  * v8.0.0-nightly.20190821
* **Operating System:**
  * Ubuntu 19.04 x64
* **Last Known Working Electron version:**
  * Unknown. Broken in `4-1-x` upwards.

### Expected Behavior

Passing additional schemes on the command line, e.g. `--standard-schemes=foo,bar,mum`, should not affect other schemes, e.g. cors enabled schemes or secure schemes.

### Actual Behavior

The schemes from earlier checks bleed into all the following switches checked.

This happens due to the "splitted" variable in `shell/app/atom_content_client.cc`'s `AtomContentClient::AddAdditionalSchemes()` being reused for each switch without being cleared first.

### To Reproduce

On the command line, add schemes for one of the argument switches handled in `AtomContentClient::AddAdditionalSchemes()`.

shell/app/atom_content_client.cc
session_types_supported->insert(media::CdmSessionType::kTemporary);
#if defined(OS_CHROMEOS)
    session_types_supported->insert(media::CdmSessionType::kPersistentLicense);
#endif  
    modes_supported->insert(media::EncryptionMode::kCenc);
    return true;
  }
  return false;
}
#endif  
#if BUILDFLAG(ENABLE_PEPPER_FLASH)
content::PepperPluginInfo CreatePepperFlashInfo(const base::FilePath& path,
                                                const std::string& version) {
  content::PepperPluginInfo plugin;
  plugin.is_out_of_process = true;
  plugin.name = content::kFlashPluginName;
  plugin.path = path;
  plugin.permissions = ppapi::PERMISSION_ALL_BITS;
  std::vector<std::string> flash_version_numbers = base::SplitString(
      version, ".", base::TRIM_WHITESPACE, base::SPLIT_WANT_NONEMPTY);
  if (flash_version_numbers.empty())
    flash_version_numbers.push_back("11");
  
  else if (flash_version_numbers[0].empty())
    flash_version_numbers[0] = "11";
  if (flash_version_numbers.size() < 2)
    flash_version_numbers.push_back("2");
  if (flash_version_numbers.size() < 3)
    flash_version_numbers.push_back("999");