### 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
When setting nativeTheme.themeSource to 'dark' (or 'system' and setting the system theme to 'dark'), the title bar on Windows remains light. On macOS, the title bar accurately reflects the setting.

* **Electron Version:**
  * `npm ./node_modules/.bin/electron --version`: 6.9.0
  * package.json: `"electron": "^8.2.5",`
  * yarn.lock: 8.2.5
* **Operating System:**
  * Windows 10 (1903)<!-- (Platform and Version) e.g. macOS 10.13.6 / Windows 10 (1803) / Ubuntu 18.04 x64 -->

### Expected Behavior
<!-- A clear and concise description of what you expected to happen. -->
(This works as expected on macOS)
|             | nativeTheme.themeSource = 'system' | nativeTheme.themeSource = 'light' | nativeTheme.themeSource = 'dark' |
|-------------|------------------------------------|-----------------------------------|----------------------------------|
| Light Theme | Light Title Bar                    | Light Title Bar                   | Dark Title Bar                   |
| Dark Theme  | Dark Title Bar                     | Light Title Bar                   | Dark Title Bar                   |

### Actual Behavior
<!-- A clear and concise description of what actually happened. -->
|  | nativeTheme.themeSource = 'system' | nativeTheme.themeSource = 'light' | nativeTheme.themeSource = 'dark' |
|-------------|------------------------------------|-----------------------------------|----------------------------------|
| Light Theme | Light Title Bar | Light Title Bar | Light Title Bar |
| Dark Theme | Light Title Bar | Light Title Bar | Light Title Bar |

### To Reproduce
<!--
Your best chance of getting this bug looked at quickly is to provide an example.
-->
```js
nativeTheme.themeSource = 'dark'
```
<!--
For bugs that can be encapsulated in a small experiment, you can use Electron Fiddle (https://github.com/electron/fiddle) to publish your example to a GitHub Gist and link it your bug report.
-->

<!--
If Fiddle is insufficient to produce an example, please provide an example REPOSITORY that can be cloned and run. You can fork electron-quick-start (https://github.com/electron/electron-quick-start) and include a link to the branch with your changes.
-->

<!--
If you provide a URL, please list the commands required to clone/setup/run your repo e.g.
```sh
$ git clone $YOUR_URL -b $BRANCH
$ npm install
$ npm start || electron .
```
-->

### Screenshots
<!-- If applicable, add screenshots to help explain your problem. -->
Command Prompt:
![Command Prompt](https://user-images.githubusercontent.com/4406098/81472203-533a8280-91ab-11ea-9a9d-a3c2229d08ba.png)

File Explorer:
![File Explorer](https://user-images.githubusercontent.com/4406098/81472549-95fd5a00-91ad-11ea-9a4a-7ceef2c5a5a9.png)

Title Bar in Electron with `nativeTheme.themeSource = 'dark'`
![Windows Title Bar](https://user-images.githubusercontent.com/4406098/81472609-1459fc00-91ae-11ea-936a-b0cc0c18a20e.png)
shell/common/api/features.cc
#include "electron/buildflags/buildflags.h"
#include "electron/fuses.h"
#include "printing/buildflags/buildflags.h"
#include "shell/common/gin_helper/dictionary.h"
#include "shell/common/node_includes.h"
namespace {
bool IsBuiltinSpellCheckerEnabled() {
  return BUILDFLAG(ENABLE_BUILTIN_SPELLCHECKER);
}
bool IsDesktopCapturerEnabled() {
  return BUILDFLAG(ENABLE_DESKTOP_CAPTURER);
}
bool IsOffscreenRenderingEnabled() {