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### Issue Details

* **Electron Version:**
  * 7.1.3 & 8.0.0-beta.4
* **Operating System:**
  * Linux Manjaro 18.1.3


### Expected Behavior
Electron should behave like Chromium with `prefers-color-scheme` media-queries.

### Actual Behavior
Electron always matches `prefers-color-scheme: light` so OS dark-mode isn't selected in media-query.

### To Reproduce

I made an electron-fiddle [here](https://gist.github.com/dc86e8931bef1781db8fbd0a6faf76ee). You can reproduce issue by switching to dark theme in your OS. See `<link>` tags in `<head>`.

### Screenshots
This is a comparison of same web-page in electron 7.1.3 (right) and chromium 78.0.3904.97 (left) on Arch/Manjaro. I am switching between "Adwaita-dark" and "Adwaita" themes on gnome.

![Peek 2019-12-06 20-06](https://user-images.githubusercontent.com/83857/70368820-84adcb80-1864-11ea-876a-21ad7c612c1e.gif)

shell/browser/electron_browser_main_parts.cc
DCHECK(base::ThreadTaskRunnerHandle::IsSet());
  
  
  js_env_ = std::make_unique<JavascriptEnvironment>(node_bindings_->uv_loop());
  v8::HandleScope scope(js_env_->isolate());
  node_bindings_->Initialize();
  
  node::Environment* env = node_bindings_->CreateEnvironment(
      js_env_->context(), js_env_->platform());
  node_env_ = std::make_unique<NodeEnvironment>(env);
  
  node_debugger_ = std::make_unique<NodeDebugger>(env);
  node_debugger_->Start();
  
  electron_bindings_->BindTo(js_env_->isolate(), env->process_object());
  
  node_bindings_->LoadEnvironment(env);
  
  node_bindings_->set_uv_env(env);
  
  
  
  
  base::FeatureList::ClearInstanceForTesting();