### 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
#if BUILDFLAG(ENABLE_PDF_VIEWER)
  content::PepperPluginInfo pdf_info;
  pdf_info.is_internal = true;
  pdf_info.is_out_of_process = true;
  pdf_info.name = "Chromium PDF Viewer";
  pdf_info.description = "Portable Document Format";
  pdf_info.path = base::FilePath::FromUTF8Unsafe(kPdfPluginPath);
  content::WebPluginMimeType pdf_mime_type(kPdfPluginMimeType, "pdf",
                                           "Portable Document Format");
  pdf_info.mime_types.push_back(pdf_mime_type);
  pdf_info.internal_entry_points.get_interface = chrome_pdf::PPP_GetInterface;
  pdf_info.internal_entry_points.initialize_module =
      chrome_pdf::PPP_InitializeModule;
  pdf_info.internal_entry_points.shutdown_module =
      chrome_pdf::PPP_ShutdownModule;
  pdf_info.permissions = ppapi::PERMISSION_PRIVATE | ppapi::PERMISSION_DEV;
  plugins->push_back(pdf_info);
#endif  
}
void ConvertStringWithSeparatorToVector(std::vector<std::string>* vec,