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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
.SetMethod("_setNextChildWebPreferences",
                 &WebContents::SetNextChildWebPreferences)
      .SetMethod("addWorkSpace", &WebContents::AddWorkSpace)
      .SetMethod("removeWorkSpace", &WebContents::RemoveWorkSpace)
      .SetMethod("showDefinitionForSelection",
                 &WebContents::ShowDefinitionForSelection)
      .SetMethod("copyImageAt", &WebContents::CopyImageAt)
      .SetMethod("capturePage", &WebContents::CapturePage)
      .SetMethod("setEmbedder", &WebContents::SetEmbedder)
      .SetMethod("setDevToolsWebContents", &WebContents::SetDevToolsWebContents)
      .SetMethod("getNativeView", &WebContents::GetNativeView)
      .SetMethod("incrementCapturerCount", &WebContents::IncrementCapturerCount)
      .SetMethod("decrementCapturerCount", &WebContents::DecrementCapturerCount)
      .SetMethod("isBeingCaptured", &WebContents::IsBeingCaptured)
      .SetMethod("setWebRTCIPHandlingPolicy",
                 &WebContents::SetWebRTCIPHandlingPolicy)
      .SetMethod("getWebRTCIPHandlingPolicy",
                 &WebContents::GetWebRTCIPHandlingPolicy)
      .SetMethod("_grantOriginAccess", &WebContents::GrantOriginAccess)
      .SetMethod("takeHeapSnapshot", &WebContents::TakeHeapSnapshot)
      .SetProperty("id", &WebContents::ID)
      .SetProperty("session", &WebContents::Session)
      .SetProperty("hostWebContents", &WebContents::HostWebContents)
      .SetProperty("devToolsWebContents", &WebContents::DevToolsWebContents)
      .SetProperty("debugger", &WebContents::Debugger)
      .SetProperty("_initiallyShown", &WebContents::WasInitiallyShown)
      .SetProperty("mainFrame", &WebContents::MainFrame)
      .Build();
}
const char* WebContents::GetTypeName() {