### 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
.SetMethod("setIgnoreMenuShortcuts", &WebContents::SetIgnoreMenuShortcuts)
      .SetMethod("setAudioMuted", &WebContents::SetAudioMuted)
      .SetMethod("isAudioMuted", &WebContents::IsAudioMuted)
      .SetMethod("isCurrentlyAudible", &WebContents::IsCurrentlyAudible)
      .SetMethod("undo", &WebContents::Undo)
      .SetMethod("redo", &WebContents::Redo)
      .SetMethod("cut", &WebContents::Cut)
      .SetMethod("copy", &WebContents::Copy)
      .SetMethod("paste", &WebContents::Paste)
      .SetMethod("pasteAndMatchStyle", &WebContents::PasteAndMatchStyle)
      .SetMethod("delete", &WebContents::Delete)
      .SetMethod("selectAll", &WebContents::SelectAll)
      .SetMethod("unselect", &WebContents::Unselect)
      .SetMethod("replace", &WebContents::Replace)
      .SetMethod("replaceMisspelling", &WebContents::ReplaceMisspelling)
      .SetMethod("findInPage", &WebContents::FindInPage)
      .SetMethod("stopFindInPage", &WebContents::StopFindInPage)
      .SetMethod("focus", &WebContents::Focus)
      .SetMethod("isFocused", &WebContents::IsFocused)
      .SetMethod("sendInputEvent", &WebContents::SendInputEvent)
      .SetMethod("beginFrameSubscription", &WebContents::BeginFrameSubscription)
      .SetMethod("endFrameSubscription", &WebContents::EndFrameSubscription)
      .SetMethod("startDrag", &WebContents::StartDrag)
      .SetMethod("attachToIframe", &WebContents::AttachToIframe)
      .SetMethod("detachFromOuterFrame", &WebContents::DetachFromOuterFrame)
      .SetMethod("isOffscreen", &WebContents::IsOffScreen)
#if BUILDFLAG(ENABLE_OSR)
      .SetMethod("startPainting", &WebContents::StartPainting)
      .SetMethod("stopPainting", &WebContents::StopPainting)
      .SetMethod("isPainting", &WebContents::IsPainting)