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### Issue Details

* **Electron Version:**
  * 12.0.0-beta.21, though as far back as 5.0.13
* **Operating System:**
  * Windows 10
* **Last Known Working Electron version:**
  * N/A

### Expected Behavior
1. Put an iframe inside of a webview
2. Call requestFullScreen on the inner iframe
3. Expect the iframe to expand and take up the entire window

### Actual Behavior
For step 3, neither the iframe nor the webview expand fully. Only the window itself becomes full-screen.

### To Reproduce
Check the "iframe inside webview" case of this minimal repro:
[fiddle.zip](https://github.com/electron/electron/files/5968821/fiddle.zip)

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### Screenshots
On an earlier version of the fiddle attached above:
<!-- If applicable, add screenshots to help explain your problem. -->
![electron-iframe-webview](https://user-images.githubusercontent.com/7199958/107708227-0030b700-6c78-11eb-9c2b-cddd1d0a3cee.gif)

shell/browser/api/electron_api_web_contents.cc
const ui::Cursor& cursor = webcursor.cursor();
  if (cursor.type() == ui::mojom::CursorType::kCustom) {
    Emit("cursor-changed", CursorTypeToString(cursor),
         gfx::Image::CreateFrom1xBitmap(cursor.custom_bitmap()),
         cursor.image_scale_factor(),
         gfx::Size(cursor.custom_bitmap().width(),
                   cursor.custom_bitmap().height()),
         cursor.custom_hotspot());
  } else {
    Emit("cursor-changed", CursorTypeToString(cursor));
  }
}
bool WebContents::IsGuest() const {
  return type_ == Type::kWebView;
}
void WebContents::AttachToIframe(content::WebContents* embedder_ontents,
                                 int embedder_frame_id) {
  attached_ = true;
  if (guest_delegate_)
    guest_delegate_->AttachToIframe(embedder_ontents, embedder_frame_id);
}
bool WebContents::IsOffScreen() const {
#if BUILDFLAG(ENABLE_OSR)
  return type_ == Type::kOffScreen;
#else
  return false;
#endif
}
#if BUILDFLAG(ENABLE_OSR)
void WebContents::OnPaint(const gfx::Rect& dirty_rect, const SkBitmap& bitmap) {