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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.h
#ifndef SHELL_BROWSER_ELECTRON_BROWSER_MAIN_PARTS_H_
#define SHELL_BROWSER_ELECTRON_BROWSER_MAIN_PARTS_H_
#include <list>
#include <memory>
#include <string>
#include "base/callback.h"
#include "base/metrics/field_trial.h"
#include "base/timer/timer.h"
#include "content/public/browser/browser_context.h"
#include "content/public/browser/browser_main_parts.h"
#include "content/public/common/main_function_params.h"
#include "electron/buildflags/buildflags.h"
#include "mojo/public/cpp/bindings/remote.h"
#include "services/device/public/mojom/geolocation_control.mojom.h"
#include "ui/views/layout/layout_provider.h"
class BrowserProcess;