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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
gfx::Rect rect;
  gin_helper::Promise<gfx::Image> promise(args->isolate());
  v8::Local<v8::Promise> handle = promise.GetHandle();
   get rect arguments if they exist
  args->GetNext(&rect);
  auto* const view = ontents()->GetRenderWidgetHostView();
  if (!view) {
    promise.Resolve(gfx::Image());
    return handle;