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
mate::Dictionary dict = mate::Dictionary::CreateEmpty(isolate);
  dict.Set("inError",
      !!(mediaFlags & blink::WebContextMenuData::MediaInError));
  dict.Set("isPaused",
      !!(mediaFlags & blink::WebContextMenuData::MediaPaused));
  dict.Set("isMuted",
      !!(mediaFlags & blink::WebContextMenuData::MediaMuted));
  dict.Set("hasAudio",
      !!(mediaFlags & blink::WebContextMenuData::MediaHasAudio));
  dict.Set("isLooping",
      (mediaFlags & blink::WebContextMenuData::MediaLoop) != 0);
  dict.Set("isControlsVisible",
      (mediaFlags & blink::WebContextMenuData::MediaControls) != 0);
  dict.Set("canToggleControls",
      !!(mediaFlags & blink::WebContextMenuData::MediaCanToggleControls));
  dict.Set("canRotate",
      !!(mediaFlags & blink::WebContextMenuData::MediaCanRotate));
  return mate::ConvertToV8(isolate, dict);
}
v8::Local<v8::Value> Converter<blink::WebCache::ResourceTypeStat>::ToV8(