### 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
namespace {
#if defined(WIDEVINE_CDM_AVAILABLE)
bool IsWidevineAvailable(
    base::FilePath* cdm_path,
    std::vector<media::VideoCodec>* codecs_supported,
    base::flat_set<media::CdmSessionType>* session_types_supported,
    base::flat_set<media::EncryptionMode>* modes_supported) {
  static enum {
    NOT_CHECKED,
    FOUND,
    NOT_FOUND,
  } widevine_cdm_file_check = NOT_CHECKED;
  if (widevine_cdm_file_check == NOT_CHECKED) {
    base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
    *cdm_path = command_line->GetSwitchValuePath(switches::kWidevineCdmPath);
    if (!cdm_path->empty()) {
      *cdm_path = cdm_path->AppendASCII(
          base::GetNativeLibraryName(kWidevineCdmLibraryName));
      widevine_cdm_file_check = base::PathExists(*cdm_path) ? FOUND : NOT_FOUND;
    }
  }
  if (widevine_cdm_file_check == FOUND) {
    
    
    codecs_supported->push_back(media::VideoCodec::kCodecVP8);
    codecs_supported->push_back(media::VideoCodec::kCodecVP9);
#if BUILDFLAG(USE_PROPRIETARY_CODECS)
    codecs_supported->push_back(media::VideoCodec::kCodecH264);
#endif