### 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
#include "shell/app/atom_content_client.h"
#include <string>
#include <vector>
#include "base/command_line.h"
#include "base/files/file_util.h"
#include "base/path_service.h"
#include "base/strings/string_split.h"
#include "base/strings/string_util.h"
#include "base/strings/utf_string_conversions.h"
#include "content/public/common/content_constants.h"
#include "content/public/common/pepper_plugin_info.h"
#include "electron/buildflags/buildflags.h"
#include "ppapi/shared_impl/ppapi_permissions.h"
#include "shell/browser/atom_paths.h"
#include "shell/common/options_switches.h"
#include "ui/base/l10n/l10n_util.h"
#include "ui/base/resource/resource_bundle.h"
#include "url/url_constants.h"
#include "widevine_cdm_version.h"  
#if defined(WIDEVINE_CDM_AVAILABLE)
#include "base/native_library.h"
#include "content/public/common/cdm_info.h"
#include "media/base/video_codecs.h"
#endif  
#if BUILDFLAG(ENABLE_PDF_VIEWER)
#include "pdf/pdf.h"
#include "shell/common/atom_constants.h"
#endif  
namespace electron {