### 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
const char* separator,
                                        const char* cmd_switch) {
  auto* command_line = base::CommandLine::ForCurrentProcess();
  auto string_with_separator = command_line->GetSwitchValueASCII(cmd_switch);
  if (!string_with_separator.empty())
    *vec = base::SplitString(string_with_separator, separator,
                             base::TRIM_WHITESPACE, base::SPLIT_WANT_NONEMPTY);
}
}  
AtomContentClient::AtomContentClient() {}
AtomContentClient::~AtomContentClient() {}
base::string16 AtomContentClient::GetLocalizedString(int message_id) {
  return l10n_util::GetStringUTF16(message_id);
}
base::StringPiece AtomContentClient::GetDataResource(
    int resource_id,
    ui::ScaleFactor scale_factor) {
  return ui::ResourceBundle::GetSharedInstance().GetRawDataResourceForScale(
      resource_id, scale_factor);
}
gfx::Image& AtomContentClient::GetNativeImageNamed(int resource_id) {
  return ui::ResourceBundle::GetSharedInstance().GetNativeImageNamed(
      resource_id);
}
base::RefCountedMemory* AtomContentClient::GetDataResourceBytes(