### 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
std::vector<media::CdmHostFilePath>* cdm_host_file_paths) {
  if (cdms) {
#if defined(WIDEVINE_CDM_AVAILABLE)
    base::FilePath cdm_path;
    std::vector<media::VideoCodec> video_codecs_supported;
    base::flat_set<media::CdmSessionType> session_types_supported;
    base::flat_set<media::EncryptionMode> encryption_modes_supported;
    if (IsWidevineAvailable(&cdm_path, &video_codecs_supported,
                            &session_types_supported,
                            &encryption_modes_supported)) {
      base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
      auto cdm_version_string =
          command_line->GetSwitchValueASCII(switches::kWidevineCdmVersion);
      
      
      
      const base::Version version(cdm_version_string);
      DCHECK(version.IsValid());
      content::CdmCapability capability(
          video_codecs_supported, encryption_modes_supported,
          session_types_supported, base::flat_set<media::CdmProxy::Protocol>());
      cdms->push_back(content::CdmInfo(
          kWidevineCdmDisplayName, kWidevineCdmGuid, version, cdm_path,
          kWidevineCdmFileSystemId, capability, kWidevineKeySystem, false));
    }
#endif  
  }
}
}