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* Electron version: 1.8.4
* Operating system: Windows

### Expected behavior

Off screen windows should respond to the `.focus()` method being called by making the content appear to be focused.

### Actual behavior

Currently the undocumented method `focusOnWebView()` method must be called in order to "focus" an offscreen window.

This behaviour should either be documented [in the off-screen rendering guide](https://electronjs.org/docs/tutorial/offscreen-rendering), or the `.focus()` method should fix this

### How to reproduce

* Create an offscreen window, and have its content output to some location (In my case an OpenGL texture)
* Go to a web page, such as google
* Observe no caret in focused text box, and selected text is the wrong colour

Not sure if this is just a documentation issue or a bug... Feel free to resolve as if it were working as intended and document this issue in the off-screen rendering guide.

atom/browser/api/atom_api_browser_window.cc
ResetBrowserView();
  mate::Handle<BrowserView> browser_view;
  if (value->IsNull() || value->IsUndefined()) {
    window_->SetBrowserView(nullptr);
  } else if (mate::ConvertFromV8(isolate(), value, &browser_view)) {
    window_->SetBrowserView(browser_view->view());
    browser_view->web_contents()->SetOwnerWindow(window_.get());
    browser_view_.Reset(isolate(), value);
#if defined(OS_MACOSX)
    UpdateDraggableRegions(nullptr, draggable_regions_);
#endif
  }
}
void BrowserWindow::ResetBrowserView() {
  if (browser_view_.IsEmpty()) {
    return;
  }
  mate::Handle<BrowserView> browser_view;
  if (mate::ConvertFromV8(isolate(), GetBrowserView(), &browser_view)
    && !browser_view.IsEmpty()) {
    browser_view->web_contents()->SetOwnerWindow(nullptr);
  }
  browser_view_.Reset();
}
bool BrowserWindow::IsModal() const {