### Preflight Checklist

- [X] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/main/CONTRIBUTING.md) for this project.
- [X] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/main/CODE_OF_CONDUCT.md) that this project adheres to.
- [X] I have searched the [issue tracker](https://www.github.com/electron/electron/issues) for a feature request that matches the one I want to file, without success.

### Electron Version

14.2.0 / 15.3.0 / 16.0.0-beta.6

### What operating system are you using?

macOS

### Operating System Version

macOS Catalina 10.15.7

### What arch are you using?

x64

### Last Known Working Electron version

14.0.0 / 15.0.0-beta.2

### Expected Behavior

`<webview>` element should support trasparency. Check the included Fiddle sample.

### Actual Behavior

`<webview>` element transparency is broken

### Testcase Gist URL

_No response_

### Additional Information
[webview-transparency.zip](https://github.com/electron/electron/files/7464785/webview-transparency.zip)

<img width="912" alt="Screen Shot 2021-11-03 at 1 55 47 AM" src="https://user-images.githubusercontent.com/1281234/139971477-2077f353-c17c-417f-9f13-9c5c52139efa.png">
<img width="912" alt="Screen Shot 2021-11-03 at 1 55 53 AM" src="https://user-images.githubusercontent.com/1281234/139971481-49f8afa5-3024-46ba-9339-33c492ba3677.png">

Related to:
https://github.com/electron/electron/blob/2a2a1a834c7072226e9368f36fe0d22a125803f0/shell/browser/api/electron_api_web_contents.cc#L1461

It regressed in this ranges:
- https://github.com/electron/electron/compare/v14.0.0...v14.0.1 (probably #30819)
- https://github.com/electron/electron/compare/v15.0.0-beta.2...v15.0.0-beta.3 (probably #30777)
shell/browser/api/electron_api_web_contents.cc
#include "shell/browser/ontents_preferences.h"
#include "shell/browser/ontents_zoom_controller.h"
#include "shell/browser/web_view_guest_delegate.h"
#include "shell/browser/web_view_manager.h"
#include "shell/common/api/electron_api_native_image.h"
#include "shell/common/api/electron_bindings.h"
#include "shell/common/color_util.h"
#include "shell/common/electron_constants.h"
#include "shell/common/gin_converters/base_converter.h"
#include "shell/common/gin_converters/blink_converter.h"
#include "shell/common/gin_converters/callback_converter.h"
#include "shell/common/gin_converters/content_converter.h"
#include "shell/common/gin_converters/file_path_converter.h"
#include "shell/common/gin_converters/frame_converter.h"
#include "shell/common/gin_converters/gfx_converter.h"
#include "shell/common/gin_converters/gurl_converter.h"
#include "shell/common/gin_converters/image_converter.h"
#include "shell/common/gin_converters/net_converter.h"
#include "shell/common/gin_converters/value_converter.h"
#include "shell/common/gin_helper/dictionary.h"
#include "shell/common/gin_helper/object_template_builder.h"
#include "shell/common/language_util.h"
#include "shell/common/mouse_util.h"
#include "shell/common/node_includes.h"
#include "shell/common/options_switches.h"
#include "shell/common/process_util.h"
#include "shell/common/v8_value_serializer.h"
#include "storage/browser/file_system/isolated_context.h"
#include "third_party/abseil-cpp/absl/types/optional.h"
#include "third_party/blink/public/common/associated_interfaces/associated_interface_provider.h"