Embedding arbitrary web content is problematic when it comes to keyboard shortcuts because:
- Web content can steal app shortcuts (see e.g. brave/browser-laptop#4407)
- Blocked web content (e.g. a focused `<webview>` performing expensive computation) will also prevent app shortcuts from firing immediately

To fix this, we need a way to _always_ handle certain shortcuts in the main process. I think the most flexible solution would be something like this:

``` js
app.addProtectedShortcut('CmdOrCtrl+Tab', (event) => {
  event.preventDefault();
});
```

We could then implement:

``` cpp
bool CommonWebContentsDelegate::PreHandleKeyboardEvent(
    content::WebContents* source,
    const content::NativeWebKeyboardEvent& event,
    bool* is_keyboard_shortcut) {
  return /* event is protected shortcut and the handler called preventDefault */;
}
```

@zcbenz Thoughts? I'm happy to work on this if we come up with a reasonable approach.

atom/common/native_mate_converters/blink_converter.cc
v8::Isolate* isolate, v8::Local<v8::Value> val,
    blink::WebDeviceEmulationParams* out) {
  mate::Dictionary dict;
  if (!ConvertFromV8(isolate, val, &dict))
    return false;
  std::string screen_position;
  if (dict.Get("screenPosition", &screen_position)) {
    screen_position = base::ToLowerASCII(screen_position);
    if (screen_position == "mobile")
      out->screenPosition = blink::WebDeviceEmulationParams::Mobile;
    else if (screen_position == "desktop")
      out->screenPosition = blink::WebDeviceEmulationParams::Desktop;
    else
      return false;
  }
  dict.Get("screenSize", &out->screenSize);
  dict.Get("viewPosition", &out->viewPosition);
  dict.Get("deviceScaleFactor", &out->deviceScaleFactor);
  dict.Get("viewSize", &out->viewSize);
  dict.Get("fitToView", &out->fitToView);
  dict.Get("offset", &out->offset);
  dict.Get("scale", &out->scale);
  return true;
}
bool Converter<blink::WebFindOptions>::FromV8(