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
blink::WebInputEvent::Type* out) {
    std::string type = base::ToLowerASCII(V8ToString(val));
    if (type == "mousedown")
      *out = blink::WebInputEvent::MouseDown;
    else if (type == "mouseup")
      *out = blink::WebInputEvent::MouseUp;
    else if (type == "mousemove")
      *out = blink::WebInputEvent::MouseMove;
    else if (type == "mouseenter")
      *out = blink::WebInputEvent::MouseEnter;
    else if (type == "mouseleave")
      *out = blink::WebInputEvent::MouseLeave;
    else if (type == "contextmenu")
      *out = blink::WebInputEvent::ContextMenu;
    else if (type == "mousewheel")
      *out = blink::WebInputEvent::MouseWheel;
    else if (type == "keydown")
      *out = blink::WebInputEvent::RawKeyDown;
    else if (type == "keyup")
      *out = blink::WebInputEvent::KeyUp;
    else if (type == "char")
      *out = blink::WebInputEvent::Char;
    else if (type == "touchstart")
      *out = blink::WebInputEvent::TouchStart;