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::WebMouseWheelEvent* out) {
  mate::Dictionary dict;
  if (!ConvertFromV8(isolate, val, &dict))
    return false;
  if (!ConvertFromV8(isolate, val, static_cast<blink::WebMouseEvent*>(out)))
    return false;
  dict.Get("deltaX", &out->deltaX);
  dict.Get("deltaY", &out->deltaY);
  dict.Get("wheelTicksX", &out->wheelTicksX);
  dict.Get("wheelTicksY", &out->wheelTicksY);
  dict.Get("accelerationRatioX", &out->accelerationRatioX);
  dict.Get("accelerationRatioY", &out->accelerationRatioY);
  dict.Get("hasPreciseScrollingDeltas", &out->hasPreciseScrollingDeltas);
#if defined(USE_AURA)
  
  bool can_scroll = true;
  if (dict.Get("canScroll", &can_scroll) && !can_scroll) {
    out->hasPreciseScrollingDeltas = false;
    out->modifiers &= ~blink::WebInputEvent::ControlKey;
  }
#endif
  return true;
}
bool Converter<blink::WebFloatPoint>::FromV8(