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### Issue Details

* **Electron Version:**
  * 11.3.0 / 12.0.0-beta28
* **Operating System:**
  * macOS 10.15.7
* **Last Known Working Electron version:**
  * None

### Expected Behavior

Chrome supports the `--force-fieldrials` commadn line switch in order to force certain field trials. Passing them as follows should work: `app.commandLine.appendSwitch('force-fieldtrials', 'WebRTC-Audio-Red-For-Opus/Enabled/');`

### Actual Behavior

This doesn't seem to work in Electron. The field trial is not activated.

### To Reproduce

Check the following fiddle: https://gist.github.com/saghul/a11bc277523e55d0401c7ca4b418483d it should print true.

This can be verified by launching Chrome with `--force-fieldtrials=WebRTC-Audio-Red-For-Opus/Enabled/` and checking the output of `RTCRtpReceiver.getCapabilities("audio").codecs` - it should contain an item with `mimeType` set to `audio/red`.

### Screenshots

N/A

### Additional Information

https://github.com/electron/electron/issues/25529 and https://github.com/electron/electron/issues/13731 reported the same problem, but they are closed with no way to validate if Electron is behaving correctly. The fiddle in this issue allows to validate if field trials are properly processed.
shell/browser/feature_list.cc
auto* cmd_line = base::CommandLine::ForCurrentProcess();
  auto enable_features =
      cmd_line->GetSwitchValueASCII(::switches::kEnableFeatures);
  auto disable_features =
      cmd_line->GetSwitchValueASCII(::switches::kDisableFeatures);
  
  
  
  
  disable_features +=
      std::string(",") + features::kSpareRendererForSitePerProcess.name +
      
      
      
      
      std::string(",") + net::features::kSameSiteByDefaultCookies.name +
      std::string(",") +
      net::features::kCookiesWithoutSameSiteMustBeSecure.name;
  
  
  
  
  enable_features += std::string(",") + "WebComponentsV0Enabled";
#if !BUILDFLAG(ENABLE_PICTURE_IN_PICTURE)
  disable_features += std::string(",") + media::kPictureInPicture.name;
#endif
  base::FeatureList::InitializeInstance(enable_features, disable_features);
}
}