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 type = blink::WebInputEvent::Undefined;
  mate::Dictionary dict;
  ConvertFromV8(isolate, val, &dict) && dict.Get("type", &type);
  return type;
}
bool Converter<blink::WebInputEvent>::FromV8(
    v8::Isolate* isolate, v8::Local<v8::Value> val,
    blink::WebInputEvent* out) {
  mate::Dictionary dict;
  if (!ConvertFromV8(isolate, val, &dict))
    return false;
  if (!dict.Get("type", &out->type))
    return false;
  std::vector<blink::WebInputEvent::Modifiers> modifiers;
  if (dict.Get("modifiers", &modifiers))
    out->modifiers = VectorToBitArray(modifiers);
  out->timeStampSeconds = base::Time::Now().ToDoubleT();
  return true;
}
bool Converter<blink::WebKeyboardEvent>::FromV8(
    v8::Isolate* isolate, v8::Local<v8::Value> val,
    blink::WebKeyboardEvent* out) {
  mate::Dictionary dict;
  if (!ConvertFromV8(isolate, val, &dict))
    return false;
  if (!ConvertFromV8(isolate, val, static_cast<blink::WebInputEvent*>(out)))
    return false;
  std::string str;
  if (!dict.Get("keyCode", &str))