### 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<content::PepperPluginInfo>* plugins) {
#if BUILDFLAG(ENABLE_PEPPER_FLASH)
  base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
  AddPepperFlashFromCommandLine(command_line, plugins);
#endif  
  ComputeBuiltInPlugins(plugins);
}
void AtomContentClient::AddContentDecryptionModules(
    std::vector<content::CdmInfo>* cdms,