### 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 (flash_version_numbers.size() < 4)
    flash_version_numbers.push_back("999");
  
  plugin.description = plugin.name + " " + flash_version_numbers[0] + "." +
                       flash_version_numbers[1] + " r" +
                       flash_version_numbers[2];
  plugin.version = base::JoinString(flash_version_numbers, ".");
  content::WebPluginMimeType swf_mime_type(content::kFlashPluginSwfMimeType,
                                           content::kFlashPluginSwfExtension,
                                           content::kFlashPluginSwfDescription);
  plugin.mime_types.push_back(swf_mime_type);
  content::WebPluginMimeType spl_mime_type(content::kFlashPluginSplMimeType,
                                           content::kFlashPluginSplExtension,
                                           content::kFlashPluginSplDescription);
  plugin.mime_types.push_back(spl_mime_type);
  return plugin;
}
void AddPepperFlashFromCommandLine(
    base::CommandLine* command_line,
    std::vector<content::PepperPluginInfo>* plugins) {
  base::FilePath flash_path =
      command_line->GetSwitchValuePath(switches::kPpapiFlashPath);
  if (flash_path.empty())
    return;
  auto flash_version =
      command_line->GetSwitchValueASCII(switches::kPpapiFlashVersion);
  plugins->push_back(CreatePepperFlashInfo(flash_path, flash_version));
}
#endif  
void ComputeBuiltInPlugins(std::vector<content::PepperPluginInfo>* plugins) {