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,
    content::NativeWebKeyboardEvent* out) {
  mate::Dictionary dict;
  if (!ConvertFromV8(isolate, val, &dict))
    return false;
  if (!ConvertFromV8(isolate, val, static_cast<blink::WebKeyboardEvent*>(out)))
    return false;
  dict.Get("skipInBrowser", &out->skip_in_browser);
  return true;
}
bool Converter<blink::WebMouseEvent>::FromV8(
    v8::Isolate* isolate, v8::Local<v8::Value> val, blink::WebMouseEvent* out) {
  mate::Dictionary dict;
  if (!ConvertFromV8(isolate, val, &dict))
    return false;
  if (!ConvertFromV8(isolate, val, static_cast<blink::WebInputEvent*>(out)))
    return false;
  if (!dict.Get("x", &out->x) || !dict.Get("y", &out->y))
    return false;
  if (!dict.Get("button", &out->button))
    out->button = blink::WebMouseEvent::Button::ButtonLeft;
  dict.Get("globalX", &out->globalX);
  dict.Get("globalY", &out->globalY);
  dict.Get("movementX", &out->movementX);
  dict.Get("movementY", &out->movementY);
  dict.Get("clickCount", &out->clickCount);
  return true;
}
bool Converter<blink::WebMouseWheelEvent>::FromV8(