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### Preflight Checklist
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* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

### 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
if (code != net::ERR_ABORTED) {
      EmitWarning(
          node::Environment::GetCurrent(JavascriptEnvironment::GetIsolate()),
          "Failed to load URL: " + url.possibly_invalid_spec() +
              " with error: " + description,
          "electron");
      Emit("did-fail-load", code, description, url, is_main_frame,
           frame_process_id, frame_routing_id);
    }
  }
  content::NavigationEntry* entry = navigation_handle->GetNavigationEntry();
   This check is needed due to an issue in Chromium
   Check the Chromium issue to keep updated:
   https:bugs.chromium.org/p/chromium/issues/detail?id=1178663
   If a history entry has been made and the forward/back call has been made,
   proceed with setting the new title
  if (entry && (entry->GetTransitionType() & ui::PAGE_TRANSITION_FORWARD_BACK))
    WebContents::TitleWasSet(entry);
}
void WebContents::TitleWasSet(content::NavigationEntry* entry) {
  std::u16string final_title;
  bool explicit_set = true;
  if (entry) {
    auto title = entry->GetTitle();
    auto url = entry->GetURL();
    if (url.SchemeIsFile() && title.empty()) {
      final_title = base::UTF8ToUTF16(url.ExtractFileName());
      explicit_set = false;
    } else {
      final_title = title;