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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
window_->SetClosable(closable);
}
bool BrowserWindow::IsClosable() {
  return window_->IsClosable();
}
void BrowserWindow::SetAlwaysOnTop(bool top, mate::Arguments* args) {
  std::string level = "floating";
  int relativeLevel = 0;
  std::string error;
  args->GetNext(&level);
  args->GetNext(&relativeLevel);
  window_->SetAlwaysOnTop(top, level, relativeLevel, &error);
  if (!error.empty()) {
    args->ThrowError(error);
  }
}
bool BrowserWindow::IsAlwaysOnTop() {
  return window_->IsAlwaysOnTop();
}
void BrowserWindow::Center() {
  window_->Center();
}
void BrowserWindow::SetPosition(int x, int y, mate::Arguments* args) {
  bool animate = false;
  args->GetNext(&animate);
  window_->SetPosition(gfx::Point(x, y), animate);
}
std::vector<int> BrowserWindow::GetPosition() {