* Output of `node_modules/.bin/electron --version`: **v4.0.0-beta.3**
* Operating System (Platform and Version): **Windows 10**
* Output of `node_modules/.bin/electron --version` on last known working Electron version (if applicable): **v3.0.4**

**Expected Behavior**
With electron 3, I could pass `webPreferences: {webwebSecurity: false}` to the BrowserWindow constructor (the equivalent of the `--disable-web-security` Chrome command line argument) which would disable CORB (effectively allowing cross-origin requests).

**Actual behavior**
CORB is still doing its thing (ignoring the `{webwebSecurity: false}` option; I see entries like these in the console:
`Cross-Origin Read Blocking (CORB) blocked cross-origin response <URL> with MIME type application/json. See <URL> for more details.`

**To Reproduce**
Make a request (using fetch or xhr) to a file on a different domain.
Example repo:
```
git clone  https://github.com/IAmPicard/electron-cors-repro
npm install
npm run start
```


atom/browser/atom_browser_client.cc
base::CommandLine* command_line,
    int process_id) {
  
  {
    base::FilePath child_path;
    base::PathService::Get(content::CHILD_PROCESS_EXE, &child_path);
    base::ThreadRestrictions::ScopedAllowIO allow_io;
    CHECK(base::MakeAbsoluteFilePath(command_line->GetProgram()) == child_path);
  }
  std::string process_type =
      command_line->GetSwitchValueASCII(::switches::kProcessType);
  if (process_type != ::switches::kRendererProcess)
    return;
  
  static const char* const kCommonSwitchNames[] = {switches::kStandardSchemes,
                                                   switches::kEnableSandbox,
                                                   switches::kSecureSchemes};
  command_line->CopySwitchesFrom(*base::CommandLine::ForCurrentProcess(),
                                 kCommonSwitchNames,
                                 arraysize(kCommonSwitchNames));
  
  if (!g_custom_service_worker_schemes->empty())