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
if (modifier == "shift")
      *out = blink::WebInputEvent::ShiftKey;
    else if (modifier == "control" || modifier == "ctrl")
      *out = blink::WebInputEvent::ControlKey;
    else if (modifier == "alt")
      *out = blink::WebInputEvent::AltKey;
    else if (modifier == "meta" || modifier == "command" || modifier == "cmd")
      *out = blink::WebInputEvent::MetaKey;
    else if (modifier == "iskeypad")
      *out = blink::WebInputEvent::IsKeyPad;
    else if (modifier == "isautorepeat")
      *out = blink::WebInputEvent::IsAutoRepeat;
    else if (modifier == "leftbuttondown")
      *out = blink::WebInputEvent::LeftButtonDown;
    else if (modifier == "middlebuttondown")
      *out = blink::WebInputEvent::MiddleButtonDown;
    else if (modifier == "rightbuttondown")
      *out = blink::WebInputEvent::RightButtonDown;
    else if (modifier == "capslock")
      *out = blink::WebInputEvent::CapsLockOn;
    else if (modifier == "numlock")
      *out = blink::WebInputEvent::NumLockOn;
    else if (modifier == "left")
      *out = blink::WebInputEvent::IsLeft;
    else if (modifier == "right")
      *out = blink::WebInputEvent::IsRight;
    return true;
  }
};
int GetWebInputEventType(v8::Isolate* isolate, v8::Local<v8::Value> val) {