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

### Problem Description
<!-- Is your feature request related to a problem? Please add a clear and concise description of what the problem is. -->
The `BrowserView` is much like `BrowserWindow`, but it cannot be used as a "window" for the `new-window` event.

Imagine a tabbed layout with several `BrowserView`s, much like the regular browser window. The `webPreferences.nativeWindowOpen` is enabled.

*Use-case 1:* The page inside one `BrowserView` tries to open a new window via `window.open()`. There's no way to handle `new-window` and create a new `BrowserView` such that the `window.opener` JavaScript property refers to its opener.

*Use-case 2:* A `<form target="_blank" method="POST">` inside `BrowserView` is submitted. A new `BrowserView` can be created and its `WebContents` `loadURL()` method can be used to navigate using `POST` method. But AFAIK there's no way to know what exactly `postData` that form was going to POST.

It works fine if the `new-window` is default-prevented and a new `BrowserWindow` is created with the `options.webContents` as a constructor option. But it is ignored for a `BrowserView`

### Proposed Solution
<!-- Describe the solution you'd like in a clear and concise manner -->
- Allow setting `webContents` in a `BrowserView` constructor options.
- Allow `BrowserView` to be a `e.newGuest` in a default-prevented `new-window` event.

### Alternatives Considered
<!-- A clear and concise description of any alternative solutions or features you've considered. -->

### Additional Information
<!-- Add any other context about the problem here. -->

shell/browser/api/electron_api_web_contents.cc
params.post_data);
    return nullptr;
  }
  if (!weak_this)
    return nullptr;
  content::NavigationController::LoadURLParams load_url_params(params.url);
  load_url_params.referrer = params.referrer;
  load_url_params.transition_type = params.transition;
  load_url_params.extra_headers = params.extra_headers;
  load_url_params.should_replace_current_entry =
      params.should_replace_current_entry;
  load_url_params.is_renderer_initiated = params.is_renderer_initiated;
  load_url_params.started_from_context_menu = params.started_from_context_menu;
  load_url_params.initiator_origin = params.initiator_origin;
  load_url_params.source_site_instance = params.source_site_instance;
  load_url_params.frame_tree_node_id = params.frame_tree_node_id;
  load_url_params.redirect_chain = params.redirect_chain;
  load_url_params.has_user_gesture = params.user_gesture;
  load_url_params.blob_url_loader_factory = params.blob_url_loader_factory;
  load_url_params.href_translate = params.href_translate;
  load_url_params.reload_type = params.reload_type;
  if (params.post_data) {
    load_url_params.load_type =
        content::NavigationController::LOAD_TYPE_HTTP_POST;
    load_url_params.post_data = params.post_data;
  }
  source->GetController().LoadURLWithParams(load_url_params);
  return source;
}
void WebContents::BeforeUnloadFired(content::WebContents* tab,