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::WebFloatPoint* out) {
  mate::Dictionary dict;
  if (!ConvertFromV8(isolate, val, &dict))
    return false;
  return dict.Get("x", &out->x) && dict.Get("y", &out->y);
}
bool Converter<blink::WebPoint>::FromV8(
    v8::Isolate* isolate, v8::Local<v8::Value> val, blink::WebPoint* out) {
  mate::Dictionary dict;
  if (!ConvertFromV8(isolate, val, &dict))
    return false;
  return dict.Get("x", &out->x) && dict.Get("y", &out->y);
}
bool Converter<blink::WebSize>::FromV8(
    v8::Isolate* isolate, v8::Local<v8::Value> val, blink::WebSize* out) {
  mate::Dictionary dict;
  if (!ConvertFromV8(isolate, val, &dict))
    return false;
  return dict.Get("width", &out->width) && dict.Get("height", &out->height);
}
bool Converter<blink::WebDeviceEmulationParams>::FromV8(